Fixed #18757, #14462, #21565 -- Reworked database-python type conversions

Complete rework of translating data values from database

Deprecation of SubfieldBase, removal of resolve_columns and
convert_values in favour of a more general converter based approach and
public API Field.from_db_value(). Now works seamlessly with aggregation,
.values() and raw queries.

Thanks to akaariai in particular for extensive advice and inspiration,
also to shaib, manfre and timograham for their reviews.

django/contrib/gis/db/backends/mysql/compiler.py

@@ -1,41 +0,0 @@
-from django.contrib.gis.db.models.sql.compiler import GeoSQLCompiler as BaseGeoSQLCompiler
-from django.db.backends.mysql import compiler
-
-SQLCompiler = compiler.SQLCompiler
-
-
-class GeoSQLCompiler(BaseGeoSQLCompiler, SQLCompiler):
-    def resolve_columns(self, row, fields=()):
-        """
-        Integrate the cases handled both by the base GeoSQLCompiler and the
-        main MySQL compiler (converting 0/1 to True/False for boolean fields).
-
-        Refs #15169.
-
-        """
-        row = BaseGeoSQLCompiler.resolve_columns(self, row, fields)
-        return SQLCompiler.resolve_columns(self, row, fields)
-
-
-class SQLInsertCompiler(compiler.SQLInsertCompiler, GeoSQLCompiler):
-    pass
-
-
-class SQLDeleteCompiler(compiler.SQLDeleteCompiler, GeoSQLCompiler):
-    pass
-
-
-class SQLUpdateCompiler(compiler.SQLUpdateCompiler, GeoSQLCompiler):
-    pass
-
-
-class SQLAggregateCompiler(compiler.SQLAggregateCompiler, GeoSQLCompiler):
-    pass
-
-
-class SQLDateCompiler(compiler.SQLDateCompiler, GeoSQLCompiler):
-    pass
-
-
-class SQLDateTimeCompiler(compiler.SQLDateTimeCompiler, GeoSQLCompiler):
-    pass

django/contrib/gis/db/backends/mysql/operations.py

@@ -6,7 +6,7 @@ from django.contrib.gis.db.backends.base import BaseSpatialOperations
 
 class MySQLOperations(DatabaseOperations, BaseSpatialOperations):
 
-    compiler_module = 'django.contrib.gis.db.backends.mysql.compiler'
+    compiler_module = 'django.contrib.gis.db.models.sql.compiler'
     mysql = True
     name = 'mysql'
     select = 'AsText(%s)'

django/contrib/gis/db/models/fields.py

@@ -197,6 +197,11 @@ class GeometryField(Field):
         else:
             return geom
 
+    def from_db_value(self, value, connection):
+        if value is not None:
+            value = Geometry(value)
+        return value
+
     def get_srid(self, geom):
         """
         Returns the default SRID for the given geometry, taking into account

django/contrib/gis/db/models/query.py

@@ -1,5 +1,5 @@
 from django.db import connections
-from django.db.models.query import QuerySet, ValuesQuerySet, ValuesListQuerySet
+from django.db.models.query import QuerySet
 
 from django.contrib.gis.db.models import aggregates
 from django.contrib.gis.db.models.fields import get_srid_info, PointField, LineStringField
@@ -18,19 +18,6 @@ class GeoQuerySet(QuerySet):
         super(GeoQuerySet, self).__init__(model=model, query=query, using=using, hints=hints)
         self.query = query or GeoQuery(self.model)
 
-    def values(self, *fields):
-        return self._clone(klass=GeoValuesQuerySet, setup=True, _fields=fields)
-
-    def values_list(self, *fields, **kwargs):
-        flat = kwargs.pop('flat', False)
-        if kwargs:
-            raise TypeError('Unexpected keyword arguments to values_list: %s'
-                    % (list(kwargs),))
-        if flat and len(fields) > 1:
-            raise TypeError("'flat' is not valid when values_list is called with more than one field.")
-        return self._clone(klass=GeoValuesListQuerySet, setup=True, flat=flat,
-                           _fields=fields)
-
     ### GeoQuerySet Methods ###
     def area(self, tolerance=0.05, **kwargs):
         """
@@ -767,16 +754,3 @@ class GeoQuerySet(QuerySet):
             return self.query.get_compiler(self.db)._field_column(geo_field, parent_model._meta.db_table)
         else:
             return self.query.get_compiler(self.db)._field_column(geo_field)
-
-
-class GeoValuesQuerySet(ValuesQuerySet):
-    def __init__(self, *args, **kwargs):
-        super(GeoValuesQuerySet, self).__init__(*args, **kwargs)
-        # This flag tells `resolve_columns` to run the values through
-        # `convert_values`.  This ensures that Geometry objects instead
-        # of string values are returned with `values()` or `values_list()`.
-        self.query.geo_values = True
-
-
-class GeoValuesListQuerySet(GeoValuesQuerySet, ValuesListQuerySet):
-    pass

django/contrib/gis/db/models/sql/compiler.py

@@ -1,12 +1,6 @@
-import datetime
+from django.db.backends.utils import truncate_name
-
-from django.conf import settings
-from django.db.backends.utils import truncate_name, typecast_date, typecast_timestamp
 from django.db.models.sql import compiler
-from django.db.models.sql.constants import MULTI
 from django.utils import six
-from django.utils.six.moves import zip, zip_longest
-from django.utils import timezone
 
 SQLCompiler = compiler.SQLCompiler
 
@@ -153,38 +147,13 @@ class GeoSQLCompiler(compiler.SQLCompiler):
                     col_aliases.add(field.column)
         return result, aliases
 
-    def resolve_columns(self, row, fields=()):
+    def get_converters(self, fields):
-        """
+        converters = super(GeoSQLCompiler, self).get_converters(fields)
-        This routine is necessary so that distances and geometries returned
+        for i, alias in enumerate(self.query.extra_select):
-        from extra selection SQL get resolved appropriately into Python
+            field = self.query.extra_select_fields.get(alias)
-        objects.
+            if field:
-        """
+                converters[i] = ([], [field.from_db_value], field)
-        values = []
+        return converters
-        aliases = list(self.query.extra_select)
-
-        # Have to set a starting row number offset that is used for
-        # determining the correct starting row index -- needed for
-        # doing pagination with Oracle.
-        rn_offset = 0
-        if self.connection.ops.oracle:
-            if self.query.high_mark is not None or self.query.low_mark:
-                rn_offset = 1
-        index_start = rn_offset + len(aliases)
-
-        # Converting any extra selection values (e.g., geometries and
-        # distance objects added by GeoQuerySet methods).
-        values = [self.query.convert_values(v,
-                               self.query.extra_select_fields.get(a, None),
-                               self.connection)
-                  for v, a in zip(row[rn_offset:index_start], aliases)]
-        if self.connection.ops.oracle or getattr(self.query, 'geo_values', False):
-            # We resolve the rest of the columns if we're on Oracle or if
-            # the `geo_values` attribute is defined.
-            for value, field in zip_longest(row[index_start:], fields):
-                values.append(self.query.convert_values(value, field, self.connection))
-        else:
-            values.extend(row[index_start:])
-        return tuple(values)
 
     #### Routines unique to GeoQuery ####
     def get_extra_select_format(self, alias):
@@ -268,55 +237,8 @@ class SQLAggregateCompiler(compiler.SQLAggregateCompiler, GeoSQLCompiler):
 
 
 class SQLDateCompiler(compiler.SQLDateCompiler, GeoSQLCompiler):
-    """
+    pass
-    This is overridden for GeoDjango to properly cast date columns, since
-    `GeoQuery.resolve_columns` is used for spatial values.
-    See #14648, #16757.
-    """
-    def results_iter(self):
-        if self.connection.ops.oracle:
-            from django.db.models.fields import DateTimeField
-            fields = [DateTimeField()]
-        else:
-            needs_string_cast = self.connection.features.needs_datetime_string_cast
-
-        offset = len(self.query.extra_select)
-        for rows in self.execute_sql(MULTI):
-            for row in rows:
-                date = row[offset]
-                if self.connection.ops.oracle:
-                    date = self.resolve_columns(row, fields)[offset]
-                elif needs_string_cast:
-                    date = typecast_date(str(date))
-                if isinstance(date, datetime.datetime):
-                    date = date.date()
-                yield date
 
 
 class SQLDateTimeCompiler(compiler.SQLDateTimeCompiler, GeoSQLCompiler):
-    """
+    pass
-    This is overridden for GeoDjango to properly cast date columns, since
-    `GeoQuery.resolve_columns` is used for spatial values.
-    See #14648, #16757.
-    """
-    def results_iter(self):
-        if self.connection.ops.oracle:
-            from django.db.models.fields import DateTimeField
-            fields = [DateTimeField()]
-        else:
-            needs_string_cast = self.connection.features.needs_datetime_string_cast
-
-        offset = len(self.query.extra_select)
-        for rows in self.execute_sql(MULTI):
-            for row in rows:
-                datetime = row[offset]
-                if self.connection.ops.oracle:
-                    datetime = self.resolve_columns(row, fields)[offset]
-                elif needs_string_cast:
-                    datetime = typecast_timestamp(str(datetime))
-                # Datetimes are artificially returned in UTC on databases that
-                # don't support time zone. Restore the zone used in the query.
-                if settings.USE_TZ:
-                    datetime = datetime.replace(tzinfo=None)
-                    datetime = timezone.make_aware(datetime, self.query.tzinfo)
-                yield datetime

django/contrib/gis/db/models/sql/conversion.py

@@ -1,32 +1,53 @@
 """
-This module holds simple classes used by GeoQuery.convert_values
+This module holds simple classes to convert geospatial values from the
-to convert geospatial values from the database.
+database.
 """
 
+from django.contrib.gis.geometry.backend import Geometry
+from django.contrib.gis.measure import Area, Distance
+
 
 class BaseField(object):
     empty_strings_allowed = True
 
-    def get_internal_type(self):
-        "Overloaded method so OracleQuery.convert_values doesn't balk."
-        return None
-
 
 class AreaField(BaseField):
     "Wrapper for Area values."
     def __init__(self, area_att):
         self.area_att = area_att
 
+    def from_db_value(self, value, connection):
+        if value is not None:
+            value = Area(**{self.area_att: value})
+        return value
+
+    def get_internal_type(self):
+        return 'AreaField'
+
 
 class DistanceField(BaseField):
     "Wrapper for Distance values."
     def __init__(self, distance_att):
         self.distance_att = distance_att
 
+    def from_db_value(self, value, connection):
+        if value is not None:
+            value = Distance(**{self.distance_att: value})
+        return value
+
+    def get_internal_type(self):
+        return 'DistanceField'
+
 
 class GeomField(BaseField):
     """
     Wrapper for Geometry values.  It is a lightweight alternative to
     using GeometryField (which requires an SQL query upon instantiation).
     """
-    pass
+    def from_db_value(self, value, connection):
+        if value is not None:
+            value = Geometry(value)
+        return value
+
+    def get_internal_type(self):
+        return 'GeometryField'

django/contrib/gis/db/models/sql/query.py

@@ -5,9 +5,7 @@ from django.contrib.gis.db.models.constants import ALL_TERMS
 from django.contrib.gis.db.models.fields import GeometryField
 from django.contrib.gis.db.models.lookups import GISLookup
 from django.contrib.gis.db.models.sql import aggregates as gis_aggregates
-from django.contrib.gis.db.models.sql.conversion import AreaField, DistanceField, GeomField
+from django.contrib.gis.db.models.sql.conversion import GeomField
-from django.contrib.gis.geometry.backend import Geometry
-from django.contrib.gis.measure import Area, Distance
 
 
 class GeoQuery(sql.Query):
@@ -38,32 +36,6 @@ class GeoQuery(sql.Query):
         obj.extra_select_fields = self.extra_select_fields.copy()
         return obj
 
-    def convert_values(self, value, field, connection):
-        """
-        Using the same routines that Oracle does we can convert our
-        extra selection objects into Geometry and Distance objects.
-        TODO: Make converted objects 'lazy' for less overhead.
-        """
-        if connection.ops.oracle:
-            # Running through Oracle's first.
-            value = super(GeoQuery, self).convert_values(value, field or GeomField(), connection)
-
-        if value is None:
-            # Output from spatial function is NULL (e.g., called
-            # function on a geometry field with NULL value).
-            pass
-        elif isinstance(field, DistanceField):
-            # Using the field's distance attribute, can instantiate
-            # `Distance` with the right context.
-            value = Distance(**{field.distance_att: value})
-        elif isinstance(field, AreaField):
-            value = Area(**{field.area_att: value})
-        elif isinstance(field, (GeomField, GeometryField)) and value:
-            value = Geometry(value)
-        elif field is not None:
-            return super(GeoQuery, self).convert_values(value, field, connection)
-        return value
-
     def get_aggregation(self, using, force_subq=False):
         # Remove any aggregates marked for reduction from the subquery
         # and move them to the outer AggregateQuery.

django/contrib/gis/tests/geoapp/models.py

@@ -66,3 +66,18 @@ class MinusOneSRID(models.Model):
 
     class Meta:
         app_label = 'geoapp'
+
+
+class NonConcreteField(models.IntegerField):
+
+    def db_type(self, connection):
+        return None
+
+    def get_attname_column(self):
+        attname, column = super(NonConcreteField, self).get_attname_column()
+        return attname, None
+
+
+class NonConcreteModel(NamedModel):
+    non_concrete = NonConcreteField()
+    point = models.PointField(geography=True)

django/contrib/gis/tests/geoapp/tests.py

@@ -13,9 +13,7 @@ from django.utils import six
 if HAS_GEOS:
     from django.contrib.gis.geos import (fromstr, GEOSGeometry,
         Point, LineString, LinearRing, Polygon, GeometryCollection)
-
+    from .models import Country, City, PennsylvaniaCity, State, Track, NonConcreteModel, Feature, MinusOneSRID
-    from .models import Country, City, PennsylvaniaCity, State, Track
-    from .models import Feature, MinusOneSRID
 
 
 def postgis_bug_version():
@@ -754,10 +752,5 @@ class GeoQuerySetTest(TestCase):
         self.assertEqual(None, qs.unionagg(field_name='point'))
 
     def test_non_concrete_field(self):
-        pkfield = City._meta.get_field_by_name('id')[0]
+        NonConcreteModel.objects.create(point=Point(0, 0), name='name')
-        orig_pkfield_col = pkfield.column
+        list(NonConcreteModel.objects.all())
-        pkfield.column = None
-        try:
-            list(City.objects.all())
-        finally:
-            pkfield.column = orig_pkfield_col

django/contrib/gis/tests/relatedapp/models.py

@@ -71,3 +71,8 @@ class Article(SimpleModel):
 class Book(SimpleModel):
     title = models.CharField(max_length=100)
     author = models.ForeignKey(Author, related_name='books', null=True)
+
+
+class Event(SimpleModel):
+    name = models.CharField(max_length=100)
+    when = models.DateTimeField()

django/contrib/gis/tests/relatedapp/tests.py

@@ -4,13 +4,15 @@ from django.contrib.gis.geos import HAS_GEOS
 from django.contrib.gis.tests.utils import no_oracle
 from django.db import connection
 from django.test import TestCase, skipUnlessDBFeature
+from django.test.utils import override_settings
+from django.utils import timezone
 
 if HAS_GEOS:
     from django.contrib.gis.db.models import Collect, Count, Extent, F, Union
     from django.contrib.gis.geometry.backend import Geometry
     from django.contrib.gis.geos import GEOSGeometry, Point, MultiPoint
 
-    from .models import City, Location, DirectoryEntry, Parcel, Book, Author, Article
+    from .models import City, Location, DirectoryEntry, Parcel, Book, Author, Article, Event
 
 
 @skipUnlessDBFeature("gis_enabled")
@@ -183,6 +185,12 @@ class RelatedGeoModelTest(TestCase):
             self.assertEqual(m.point, d['point'])
             self.assertEqual(m.point, t[1])
 
+    @override_settings(USE_TZ=True)
+    def test_07b_values(self):
+        "Testing values() and values_list() with aware datetime. See #21565."
+        Event.objects.create(name="foo", when=timezone.now())
+        list(Event.objects.values_list('when'))
+
     def test08_defer_only(self):
         "Testing defer() and only() on Geographic models."
         qs = Location.objects.all()

django/db/backends/__init__.py

@@ -1190,20 +1190,13 @@ class BaseDatabaseOperations(object):
             second = timezone.make_aware(second, tz)
         return [first, second]
 
-    def convert_values(self, value, field):
+    def get_db_converters(self, internal_type):
+        """Get a list of functions needed to convert field data.
+
+        Some field types on some backends do not provide data in the correct
+        format, this is the hook for coverter functions.
         """
-        Coerce the value returned by the database backend into a consistent type
+        return []
-        that is compatible with the field type.
-        """
-        if value is None or field is None:
-            return value
-        internal_type = field.get_internal_type()
-        if internal_type == 'FloatField':
-            return float(value)
-        elif (internal_type and (internal_type.endswith('IntegerField')
-                                 or internal_type == 'AutoField')):
-            return int(value)
-        return value
 
     def check_aggregate_support(self, aggregate_func):
         """Check that the backend supports the provided aggregate

django/db/backends/mysql/base.py

@@ -394,6 +394,17 @@ class DatabaseOperations(BaseDatabaseOperations):
             return 'POW(%s)' % ','.join(sub_expressions)
         return super(DatabaseOperations, self).combine_expression(connector, sub_expressions)
 
+    def get_db_converters(self, internal_type):
+        converters = super(DatabaseOperations, self).get_db_converters(internal_type)
+        if internal_type in ['BooleanField', 'NullBooleanField']:
+            converters.append(self.convert_booleanfield_value)
+        return converters
+
+    def convert_booleanfield_value(self, value, field):
+        if value in (0, 1):
+            value = bool(value)
+        return value
+
 
 class DatabaseWrapper(BaseDatabaseWrapper):
     vendor = 'mysql'

django/db/backends/mysql/compiler.py

@@ -1,18 +1,7 @@
 from django.db.models.sql import compiler
-from django.utils.six.moves import zip_longest
 
 
 class SQLCompiler(compiler.SQLCompiler):
-    def resolve_columns(self, row, fields=()):
-        values = []
-        index_extra_select = len(self.query.extra_select)
-        for value, field in zip_longest(row[index_extra_select:], fields):
-            if (field and field.get_internal_type() in ("BooleanField", "NullBooleanField") and
-                    value in (0, 1)):
-                value = bool(value)
-            values.append(value)
-        return row[:index_extra_select] + tuple(values)
-
     def as_subquery_condition(self, alias, columns, qn):
         qn2 = self.connection.ops.quote_name
         sql, params = self.as_sql()

django/db/backends/mysql/validation.py

@@ -17,6 +17,10 @@ class DatabaseValidation(BaseDatabaseValidation):
         if getattr(field, 'rel', None) is None:
             field_type = field.db_type(connection)
 
+            # Ignore any non-concrete fields
+            if field_type is None:
+                return errors
+
             if (field_type.startswith('varchar')  # Look for CharFields...
                     and field.unique  # ... that are unique
                     and (field.max_length is None or int(field.max_length) > 255)):

django/db/backends/oracle/base.py

@@ -250,49 +250,64 @@ WHEN (new.%(col_name)s IS NULL)
             sql = field_name    # Cast to DATE removes sub-second precision.
         return sql, []
 
-    def convert_values(self, value, field):
+    def get_db_converters(self, internal_type):
-        if isinstance(value, Database.LOB):
+        converters = super(DatabaseOperations, self).get_db_converters(internal_type)
-            value = value.read()
+        if internal_type == 'TextField':
-            if field and field.get_internal_type() == 'TextField':
+            converters.append(self.convert_textfield_value)
-                value = force_text(value)
+        elif internal_type == 'BinaryField':
+            converters.append(self.convert_binaryfield_value)
+        elif internal_type in ['BooleanField', 'NullBooleanField']:
+            converters.append(self.convert_booleanfield_value)
+        elif internal_type == 'DecimalField':
+            converters.append(self.convert_decimalfield_value)
+        elif internal_type == 'DateField':
+            converters.append(self.convert_datefield_value)
+        elif internal_type == 'TimeField':
+            converters.append(self.convert_timefield_value)
+        converters.append(self.convert_empty_values)
+        return converters
 
+    def convert_empty_values(self, value, field):
         # Oracle stores empty strings as null. We need to undo this in
         # order to adhere to the Django convention of using the empty
         # string instead of null, but only if the field accepts the
         # empty string.
-        if value is None and field and field.empty_strings_allowed:
+        if value is None and field.empty_strings_allowed:
+            value = ''
             if field.get_internal_type() == 'BinaryField':
                 value = b''
-            else:
+        return value
-                value = ''
+
-        # Convert 1 or 0 to True or False
+    def convert_textfield_value(self, value, field):
-        elif value in (1, 0) and field and field.get_internal_type() in ('BooleanField', 'NullBooleanField'):
+        if isinstance(value, Database.LOB):
+            value = force_text(value.read())
+        return value
+
+    def convert_binaryfield_value(self, value, field):
+        if isinstance(value, Database.LOB):
+            value = force_bytes(value.read())
+        return value
+
+    def convert_booleanfield_value(self, value, field):
+        if value in (1, 0):
             value = bool(value)
-        # Force floats to the correct type
+        return value
-        elif value is not None and field and field.get_internal_type() == 'FloatField':
+
-            value = float(value)
+    def convert_decimalfield_value(self, value, field):
-        # Convert floats to decimals
+        if value is not None:
-        elif value is not None and field and field.get_internal_type() == 'DecimalField':
             value = backend_utils.typecast_decimal(field.format_number(value))
+        return value
+
     # cx_Oracle always returns datetime.datetime objects for
     # DATE and TIMESTAMP columns, but Django wants to see a
-        # python datetime.date, .time, or .datetime.  We use the type
+    # python datetime.date, .time, or .datetime.
-        # of the Field to determine which to cast to, but it's not
+    def convert_datefield_value(self, value, field):
-        # always available.
+        if isinstance(value, Database.Timestamp):
-        # As a workaround, we cast to date if all the time-related
+            return value.date()
-        # values are 0, or to time if the date is 1/1/1900.
+
-        # This could be cleaned a bit by adding a method to the Field
+    def convert_timefield_value(self, value, field):
-        # classes to normalize values from the database (the to_python
+        if isinstance(value, Database.Timestamp):
-        # method is used for validation and isn't what we want here).
-        elif isinstance(value, Database.Timestamp):
-            if field and field.get_internal_type() == 'DateTimeField':
-                pass
-            elif field and field.get_internal_type() == 'DateField':
-                value = value.date()
-            elif field and field.get_internal_type() == 'TimeField' or (value.year == 1900 and value.month == value.day == 1):
             value = value.time()
-            elif value.hour == value.minute == value.second == value.microsecond == 0:
-                value = value.date()
         return value
 
     def deferrable_sql(self):

django/db/backends/oracle/compiler.py

@@ -1,23 +1,7 @@
 from django.db.models.sql import compiler
-from django.utils.six.moves import zip_longest
 
 
 class SQLCompiler(compiler.SQLCompiler):
-    def resolve_columns(self, row, fields=()):
-        # If this query has limit/offset information, then we expect the
-        # first column to be an extra "_RN" column that we need to throw
-        # away.
-        if self.query.high_mark is not None or self.query.low_mark:
-            rn_offset = 1
-        else:
-            rn_offset = 0
-        index_start = rn_offset + len(self.query.extra_select)
-        values = [self.query.convert_values(v, None, connection=self.connection)
-                  for v in row[rn_offset:index_start]]
-        for value, field in zip_longest(row[index_start:], fields):
-            values.append(self.query.convert_values(value, field, connection=self.connection))
-        return tuple(values)
-
     def as_sql(self, with_limits=True, with_col_aliases=False):
         """
         Creates the SQL for this query. Returns the SQL string and list
@@ -48,7 +32,7 @@ class SQLCompiler(compiler.SQLCompiler):
             high_where = ''
             if self.query.high_mark is not None:
                 high_where = 'WHERE ROWNUM <= %d' % (self.query.high_mark,)
-            sql = 'SELECT * FROM (SELECT ROWNUM AS "_RN", "_SUB".* FROM (%s) "_SUB" %s) WHERE "_RN" > %d' % (sql, high_where, self.query.low_mark)
+            sql = 'SELECT * FROM (SELECT "_SUB".*, ROWNUM AS "_RN" FROM (%s) "_SUB" %s) WHERE "_RN" > %d' % (sql, high_where, self.query.low_mark)
 
         return sql, params
 

django/db/backends/sqlite3/base.py

@@ -263,27 +263,36 @@ class DatabaseOperations(BaseDatabaseOperations):
 
         return six.text_type(value)
 
-    def convert_values(self, value, field):
+    def get_db_converters(self, internal_type):
-        """SQLite returns floats when it should be returning decimals,
+        converters = super(DatabaseOperations, self).get_db_converters(internal_type)
-        and gets dates and datetimes wrong.
+        if internal_type == 'DateTimeField':
-        For consistency with other backends, coerce when required.
+            converters.append(self.convert_datetimefield_value)
-        """
-        if value is None:
-            return None
-
-        internal_type = field.get_internal_type()
-        if internal_type == 'DecimalField':
-            return backend_utils.typecast_decimal(field.format_number(value))
-        elif internal_type and internal_type.endswith('IntegerField') or internal_type == 'AutoField':
-            return int(value)
         elif internal_type == 'DateField':
-            return parse_date(value)
+            converters.append(self.convert_datefield_value)
-        elif internal_type == 'DateTimeField':
-            return parse_datetime_with_timezone_support(value)
         elif internal_type == 'TimeField':
-            return parse_time(value)
+            converters.append(self.convert_timefield_value)
+        elif internal_type == 'DecimalField':
+            converters.append(self.convert_decimalfield_value)
+        return converters
 
-        # No field, or the field isn't known to be a decimal or integer
+    def convert_decimalfield_value(self, value, field):
+        if value is not None:
+            value = backend_utils.typecast_decimal(field.format_number(value))
+        return value
+
+    def convert_datefield_value(self, value, field):
+        if value is not None and not isinstance(value, datetime.date):
+            value = parse_date(value)
+        return value
+
+    def convert_datetimefield_value(self, value, field):
+        if value is not None and not isinstance(value, datetime.datetime):
+            value = parse_datetime_with_timezone_support(value)
+        return value
+
+    def convert_timefield_value(self, value, field):
+        if value is not None and not isinstance(value, datetime.time):
+            value = parse_time(value)
         return value
 
     def bulk_insert_sql(self, fields, num_values):

django/db/models/fields/__init__.py

@@ -558,6 +558,11 @@ class Field(RegisterLookupMixin):
     def db_type_suffix(self, connection):
         return connection.creation.data_types_suffix.get(self.get_internal_type())
 
+    def get_db_converters(self, connection):
+        if hasattr(self, 'from_db_value'):
+            return [self.from_db_value]
+        return []
+
     @property
     def unique(self):
         return self._unique or self.primary_key

django/db/models/fields/subclassing.py

@@ -7,6 +7,10 @@ to_python() and the other necessary methods and everything will work
 seamlessly.
 """
 
+import warnings
+
+from django.utils.deprecation import RemovedInDjango20Warning
+
 
 class SubfieldBase(type):
     """
@@ -14,6 +18,9 @@ class SubfieldBase(type):
     has the descriptor protocol attached to it.
     """
     def __new__(cls, name, bases, attrs):
+        warnings.warn("SubfieldBase has been deprecated. Use Field.from_db_value instead.",
+                  RemovedInDjango20Warning)
+
         new_class = super(SubfieldBase, cls).__new__(cls, name, bases, attrs)
         new_class.contribute_to_class = make_contrib(
             new_class, attrs.get('contribute_to_class')

django/db/models/query.py

@@ -1560,7 +1560,6 @@ class RawQuerySet(object):
         compiler = connections[db].ops.compiler('SQLCompiler')(
             self.query, connections[db], db
         )
-        need_resolv_columns = hasattr(compiler, 'resolve_columns')
 
         query = iter(self.query)
 
@@ -1578,11 +1577,11 @@ class RawQuerySet(object):
                 model_cls = deferred_class_factory(self.model, skip)
             else:
                 model_cls = self.model
-            if need_resolv_columns:
             fields = [self.model_fields.get(c, None) for c in self.columns]
+            converters = compiler.get_converters(fields)
             for values in query:
-                if need_resolv_columns:
+                if converters:
-                    values = compiler.resolve_columns(values, fields)
+                    values = compiler.apply_converters(values, converters)
                 # Associate fields to values
                 model_init_values = [values[pos] for pos in model_init_pos]
                 instance = model_cls.from_db(db, model_init_names, model_init_values)

django/db/models/sql/compiler.py

@@ -690,12 +690,34 @@ class SQLCompiler(object):
         self.query.deferred_to_data(columns, self.query.deferred_to_columns_cb)
         return columns
 
+    def get_converters(self, fields):
+        converters = {}
+        index_extra_select = len(self.query.extra_select)
+        for i, field in enumerate(fields):
+            if field:
+                backend_converters = self.connection.ops.get_db_converters(field.get_internal_type())
+                field_converters = field.get_db_converters(self.connection)
+                if backend_converters or field_converters:
+                    converters[index_extra_select + i] = (backend_converters, field_converters, field)
+        return converters
+
+    def apply_converters(self, row, converters):
+        row = list(row)
+        for pos, (backend_converters, field_converters, field) in converters.items():
+            value = row[pos]
+            for converter in backend_converters:
+                value = converter(value, field)
+            for converter in field_converters:
+                value = converter(value, self.connection)
+            row[pos] = value
+        return tuple(row)
+
     def results_iter(self):
         """
         Returns an iterator over the results from executing this query.
         """
-        resolve_columns = hasattr(self, 'resolve_columns')
         fields = None
+        converters = None
         has_aggregate_select = bool(self.query.aggregate_select)
         for rows in self.execute_sql(MULTI):
             for row in rows:
@@ -703,7 +725,6 @@ class SQLCompiler(object):
                     loaded_fields = self.query.get_loaded_field_names().get(self.query.model, set()) or self.query.select
                     aggregate_start = len(self.query.extra_select) + len(loaded_fields)
                     aggregate_end = aggregate_start + len(self.query.aggregate_select)
-                if resolve_columns:
                 if fields is None:
                     # We only set this up here because
                     # related_select_cols isn't populated until
@@ -725,7 +746,7 @@ class SQLCompiler(object):
                     fields = fields + [f.field for f in self.query.related_select_cols]
 
                     # If the field was deferred, exclude it from being passed
-                        # into `resolve_columns` because it wasn't selected.
+                    # into `get_converters` because it wasn't selected.
                     only_load = self.deferred_to_columns()
                     if only_load:
                         fields = [f for f in fields if f.model._meta.db_table not in only_load or
@@ -735,7 +756,9 @@ class SQLCompiler(object):
                         fields = fields[:aggregate_start] + [
                             None for x in range(0, aggregate_end - aggregate_start)
                         ] + fields[aggregate_start:]
-                    row = self.resolve_columns(row, fields)
+                    converters = self.get_converters(fields)
+                if converters:
+                    row = self.apply_converters(row, converters)
 
                 if has_aggregate_select:
                     row = tuple(row[:aggregate_start]) + tuple(
@@ -1092,22 +1115,13 @@ class SQLDateCompiler(SQLCompiler):
         """
         Returns an iterator over the results from executing this query.
         """
-        resolve_columns = hasattr(self, 'resolve_columns')
-        if resolve_columns:
         from django.db.models.fields import DateField
-            fields = [DateField()]
+        converters = self.get_converters([DateField()])
-        else:
-            from django.db.backends.utils import typecast_date
-            needs_string_cast = self.connection.features.needs_datetime_string_cast
 
         offset = len(self.query.extra_select)
         for rows in self.execute_sql(MULTI):
             for row in rows:
-                date = row[offset]
+                date = self.apply_converters(row, converters)[offset]
-                if resolve_columns:
-                    date = self.resolve_columns(row, fields)[offset]
-                elif needs_string_cast:
-                    date = typecast_date(str(date))
                 if isinstance(date, datetime.datetime):
                     date = date.date()
                 yield date
@@ -1118,22 +1132,13 @@ class SQLDateTimeCompiler(SQLCompiler):
         """
         Returns an iterator over the results from executing this query.
         """
-        resolve_columns = hasattr(self, 'resolve_columns')
-        if resolve_columns:
         from django.db.models.fields import DateTimeField
-            fields = [DateTimeField()]
+        converters = self.get_converters([DateTimeField()])
-        else:
-            from django.db.backends.utils import typecast_timestamp
-            needs_string_cast = self.connection.features.needs_datetime_string_cast
 
         offset = len(self.query.extra_select)
         for rows in self.execute_sql(MULTI):
             for row in rows:
-                datetime = row[offset]
+                datetime = self.apply_converters(row, converters)[offset]
-                if resolve_columns:
-                    datetime = self.resolve_columns(row, fields)[offset]
-                elif needs_string_cast:
-                    datetime = typecast_timestamp(str(datetime))
                 # Datetimes are artificially returned in UTC on databases that
                 # don't support time zone. Restore the zone used in the query.
                 if settings.USE_TZ:

django/db/models/sql/query.py

@@ -54,15 +54,6 @@ class RawQuery(object):
     def clone(self, using):
         return RawQuery(self.sql, using, params=self.params)
 
-    def convert_values(self, value, field, connection):
-        """Convert the database-returned value into a type that is consistent
-        across database backends.
-
-        By default, this defers to the underlying backend operations, but
-        it can be overridden by Query classes for specific backends.
-        """
-        return connection.ops.convert_values(value, field)
-
     def get_columns(self):
         if self.cursor is None:
             self._execute_query()
@@ -308,15 +299,6 @@ class Query(object):
             obj._setup_query()
         return obj
 
-    def convert_values(self, value, field, connection):
-        """Convert the database-returned value into a type that is consistent
-        across database backends.
-
-        By default, this defers to the underlying backend operations, but
-        it can be overridden by Query classes for specific backends.
-        """
-        return connection.ops.convert_values(value, field)
-
     def resolve_aggregate(self, value, aggregate, connection):
         """Resolve the value of aggregates returned by the database to
         consistent (and reasonable) types.
@@ -337,7 +319,13 @@ class Query(object):
             return float(value)
         else:
             # Return value depends on the type of the field being processed.
-            return self.convert_values(value, aggregate.field, connection)
+            backend_converters = connection.ops.get_db_converters(aggregate.field.get_internal_type())
+            field_converters = aggregate.field.get_db_converters(connection)
+            for converter in backend_converters:
+                value = converter(value, aggregate.field)
+            for converter in field_converters:
+                value = converter(value, connection)
+            return value
 
     def get_aggregation(self, using, force_subq=False):
         """

docs/howto/custom-model-fields.txt

@@ -317,77 +317,6 @@ and reconstructing the field::
     new_instance = MyField(*args, **kwargs)
     self.assertEqual(my_field_instance.some_attribute, new_instance.some_attribute)
 
-
-The ``SubfieldBase`` metaclass
-------------------------------
-
-.. class:: django.db.models.SubfieldBase
-
-As we indicated in the introduction_, field subclasses are often needed for
-two reasons: either to take advantage of a custom database column type, or to
-handle complex Python types. Obviously, a combination of the two is also
-possible. If you're only working with custom database column types and your
-model fields appear in Python as standard Python types direct from the
-database backend, you don't need to worry about this section.
-
-If you're handling custom Python types, such as our ``Hand`` class, we need to
-make sure that when Django initializes an instance of our model and assigns a
-database value to our custom field attribute, we convert that value into the
-appropriate Python object. The details of how this happens internally are a
-little complex, but the code you need to write in your ``Field`` class is
-simple: make sure your field subclass uses a special metaclass:
-
-For example, on Python 2::
-
-    class HandField(models.Field):
-
-        description = "A hand of cards (bridge style)"
-
-        __metaclass__ = models.SubfieldBase
-
-        def __init__(self, *args, **kwargs):
-            ...
-
-On Python 3, in lieu of setting the ``__metaclass__`` attribute, add
-``metaclass`` to the class definition::
-
-    class HandField(models.Field, metaclass=models.SubfieldBase):
-        ...
-
-If you want your code to work on Python 2 & 3, you can use
-:func:`six.with_metaclass`::
-
-    from django.utils.six import with_metaclass
-
-    class HandField(with_metaclass(models.SubfieldBase, models.Field)):
-        ...
-
-This ensures that the :meth:`.to_python` method will always be called when the
-attribute is initialized.
-
-``ModelForm``\s and custom fields
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-
-If you use :class:`~django.db.models.SubfieldBase`, :meth:`.to_python` will be
-called every time an instance of the field is assigned a value (in addition to
-its usual call when retrieving the value from the database). This means that
-whenever a value may be assigned to the field, you need to ensure that it will
-be of the correct datatype, or that you handle any exceptions.
-
-This is especially important if you use :doc:`ModelForms
-</topics/forms/modelforms>`. When saving a ModelForm, Django will use
-form values to instantiate model instances. However, if the cleaned
-form data can't be used as valid input to the field, the normal form
-validation process will break.
-
-Therefore, you must ensure that the form field used to represent your
-custom field performs whatever input validation and data cleaning is
-necessary to convert user-provided form input into a
-``to_python()``-compatible model field value. This may require writing a
-custom form field, and/or implementing the :meth:`.formfield` method on
-your field to return a form field class whose ``to_python()`` returns the
-correct datatype.
-
 Documenting your custom field
 -----------------------------
 
@@ -500,59 +429,79 @@ over this field. You are then responsible for creating the column in the right
 table in some other way, of course, but this gives you a way to tell Django to
 get out of the way.
 
-.. _converting-database-values-to-python-objects:
+.. _converting-values-to-python-objects:
 
-Converting database values to Python objects
+Converting values to Python objects
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+.. versionchanged:: 1.8
+
+    Historically, Django provided a metaclass called ``SubfieldBase`` which
+    always called :meth:`~Field.to_python` on assignment. This did not play
+    nicely with custom database transformations, aggregation, or values
+    queries, so it has been replaced with :meth:`~Field.from_db_value`.
 
 If your custom :class:`~Field` class deals with data structures that are more
-complex than strings, dates, integers or floats, then you'll need to override
+complex than strings, dates, integers, or floats, then you may need to override
-:meth:`~Field.to_python`. As a general rule, the method should deal gracefully
+:meth:`~Field.from_db_value` and :meth:`~Field.to_python`.
-with any of the following arguments:
+
+If present for the field subclass, ``from_db_value()`` will be called in all
+circumstances when the data is loaded from the database, including in
+aggregates and :meth:`~django.db.models.query.QuerySet.values` calls.
+
+``to_python()`` is called by deserialization and during the
+:meth:`~django.db.models.Model.clean` method used from forms.
+
+As a general rule, ``to_python()`` should deal gracefully with any of the
+following arguments:
 
 * An instance of the correct type (e.g., ``Hand`` in our ongoing example).
 
-* A string (e.g., from a deserializer).
+* A string
 
-* Whatever the database returns for the column type you're using.
+* ``None`` (if the field allows ``null=True``)
 
 In our ``HandField`` class, we're storing the data as a VARCHAR field in the
-database, so we need to be able to process strings and ``Hand`` instances in
+database, so we need to be able to process strings and ``None`` in the
-:meth:`.to_python`::
+``from_db_value()``. In ``to_python()``, we need to also handle ``Hand``
+instances::
 
     import re
 
+    from django.core.exceptions import ValidationError
+    from django.db import models
+
+    def parse_hand(hand_string):
+        """Takes a string of cards and splits into a full hand."""
+        p1 = re.compile('.{26}')
+        p2 = re.compile('..')
+        args = [p2.findall(x) for x in p1.findall(hand_string)]
+        if len(args) != 4:
+            raise ValidationError("Invalid input for a Hand instance")
+        return Hand(*args)
+
     class HandField(models.Field):
         # ...
 
+        def from_db_value(self, value, connection):
+            if value is None:
+                return value
+            return parse_hand(value)
+
         def to_python(self, value):
             if isinstance(value, Hand):
                 return value
 
-            # The string case.
+            if value is None:
-            p1 = re.compile('.{26}')
+                return value
-            p2 = re.compile('..')
-            args = [p2.findall(x) for x in p1.findall(value)]
-            if len(args) != 4:
-                raise ValidationError("Invalid input for a Hand instance")
-            return Hand(*args)
 
-Notice that we always return a ``Hand`` instance from this method. That's the
+            return parse_hand(value)
-Python object type we want to store in the model's attribute. If anything is
-going wrong during value conversion, you should raise a
-:exc:`~django.core.exceptions.ValidationError` exception.
 
-**Remember:** If your custom field needs the :meth:`~Field.to_python` method to be
+Notice that we always return a ``Hand`` instance from these methods. That's the
-called when it is created, you should be using `The SubfieldBase metaclass`_
+Python object type we want to store in the model's attribute.
-mentioned earlier. Otherwise :meth:`~Field.to_python` won't be called
-automatically.
 
-.. warning::
+For ``to_python()``, if anything goes wrong during value conversion, you should
-
+raise a :exc:`~django.core.exceptions.ValidationError` exception.
-    If your custom field allows ``null=True``, any field method that takes
-    ``value`` as an argument, like :meth:`~Field.to_python` and
-    :meth:`~Field.get_prep_value`, should handle the case when ``value`` is
-    ``None``.
 
 .. _converting-python-objects-to-query-values:
 

docs/internals/deprecation.txt

@@ -57,6 +57,8 @@ about each item can often be found in the release notes of two versions prior.
 * The ``is_admin_site`` argument to
   ``django.contrib.auth.views.password_reset()`` will be removed.
 
+* ``django.db.models.field.subclassing.SubfieldBase`` will be removed.
+
 .. _deprecation-removed-in-1.9:
 
 1.9

docs/ref/models/fields.txt

@@ -1532,7 +1532,7 @@ Field API reference
     ``Field`` is an abstract class that represents a database table column.
     Django uses fields to create the database table (:meth:`db_type`), to map
     Python types to database (:meth:`get_prep_value`) and vice-versa
-    (:meth:`to_python`), and to apply :doc:`/ref/models/lookups`
+    (:meth:`from_db_value`), and to apply :doc:`/ref/models/lookups`
     (:meth:`get_prep_lookup`).
 
     A field is thus a fundamental piece in different Django APIs, notably,
@@ -1609,17 +1609,26 @@ Field API reference
 
         See :ref:`converting-query-values-to-database-values` for usage.
 
-    When loading data, :meth:`to_python` is used:
+    When loading data, :meth:`from_db_value` is used:
 
-    .. method:: to_python(value)
+    .. method:: from_db_value(value, connection)
 
-        Converts a value as returned by the database (or a serializer) to a
+        .. versionadded:: 1.8
-        Python object. It is the reverse of :meth:`get_prep_value`.
 
-        The default implementation returns ``value``, which is the common case
+        Converts a value as returned by the database to a Python object. It is
-        when the database backend already returns the correct Python type.
+        the reverse of :meth:`get_prep_value`.
 
-        See :ref:`converting-database-values-to-python-objects` for usage.
+        This method is not used for most built-in fields as the database
+        backend already returns the correct Python type, or the backend itself
+        does the conversion.
+
+        See :ref:`converting-values-to-python-objects` for usage.
+
+        .. note::
+        
+            For performance reasons, ``from_db_value`` is not implemented as a
+            no-op on fields which do not require it (all Django fields).
+            Consequently you may not call ``super`` in your definition.
 
     When saving, :meth:`pre_save` and :meth:`get_db_prep_save` are used:
 
@@ -1644,15 +1653,6 @@ Field API reference
 
         See :ref:`preprocessing-values-before-saving` for usage.
 
-    Besides saving to the database, the field also needs to know how to
-    serialize its value (inverse of :meth:`to_python`):
-
-    .. method:: value_to_string(obj)
-
-        Converts ``obj`` to a string. Used to serialize the value of the field.
-
-        See :ref:`converting-model-field-to-serialization` for usage.
-
     When a lookup is used on a field, the value may need to be "prepared".
     Django exposes two methods for this:
 
@@ -1682,6 +1682,26 @@ Field API reference
         ``prepared`` describes whether the value has already been prepared with
         :meth:`get_prep_lookup`.
 
+    Fields often receive their values as a different type, either from
+    serialization or from forms.
+    
+    .. method:: to_python(value)
+
+        Converts the value into the correct Python object. It acts as the
+        reverse of :meth:`value_to_string`, and is also called in
+        :meth:`~django.db.models.Model.clean`.
+
+        See :ref:`converting-values-to-python-objects` for usage.
+
+    Besides saving to the database, the field also needs to know how to
+    serialize its value:
+
+    .. method:: value_to_string(obj)
+
+        Converts ``obj`` to a string. Used to serialize the value of the field.
+
+        See :ref:`converting-model-field-to-serialization` for usage.
+
     When using :class:`model forms <django.forms.ModelForm>`, the ``Field``
     needs to know which form field it should be represented by:
 

docs/releases/1.8.txt

@@ -736,3 +736,14 @@ also been deprecated.
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 It's a legacy option that should no longer be necessary.
+
+``SubfieldBase``
+~~~~~~~~~~~~~~~~
+
+``django.db.models.fields.subclassing.SubfieldBase`` has been deprecated and
+will be removed in Django 2.0. Historically, it was used to handle fields where
+type conversion was needed when loading from the database, but it was not used
+in ``.values()`` calls or in aggregates. It has been replaced with
+:meth:`~django.db.models.Field.from_db_value`. Note that the new approach does
+not call the :meth:`~django.db.models.Fields.to_python`` method on assignment
+as was the case with ``SubfieldBase``.

tests/aggregation_regress/tests.py

@@ -894,18 +894,6 @@ class AggregationTests(TestCase):
             lambda b: b.name
         )
 
-    def test_type_conversion(self):
-        # The database backend convert_values function should not try to covert
-        # CharFields to float. Refs #13844.
-        from django.db.models import CharField
-        from django.db import connection
-        testData = 'not_a_float_value'
-        testField = CharField()
-        self.assertEqual(
-            connection.ops.convert_values(testData, testField),
-            testData
-        )
-
     def test_annotate_joins(self):
         """
         Test that the base table's join isn't promoted to LOUTER. This could

tests/backends/tests.py

@@ -20,8 +20,6 @@ from django.db.backends.signals import connection_created
 from django.db.backends.postgresql_psycopg2 import version as pg_version
 from django.db.backends.utils import format_number, CursorWrapper
 from django.db.models import Sum, Avg, Variance, StdDev
-from django.db.models.fields import (AutoField, DateField, DateTimeField,
-    DecimalField, IntegerField, TimeField)
 from django.db.models.sql.constants import CURSOR
 from django.db.utils import ConnectionHandler
 from django.test import (TestCase, TransactionTestCase, override_settings,
@@ -133,16 +131,6 @@ class SQLiteTests(TestCase):
             self.assertRaises(NotImplementedError,
                 models.Item.objects.all().aggregate, aggregate('last_modified'))
 
-    def test_convert_values_to_handle_null_value(self):
-        from django.db.backends.sqlite3.base import DatabaseOperations
-        convert_values = DatabaseOperations(connection).convert_values
-        self.assertIsNone(convert_values(None, AutoField(primary_key=True)))
-        self.assertIsNone(convert_values(None, DateField()))
-        self.assertIsNone(convert_values(None, DateTimeField()))
-        self.assertIsNone(convert_values(None, DecimalField()))
-        self.assertIsNone(convert_values(None, IntegerField()))
-        self.assertIsNone(convert_values(None, TimeField()))
-
 
 @unittest.skipUnless(connection.vendor == 'postgresql', "Test only for PostgreSQL")
 class PostgreSQLTests(TestCase):

tests/custom_pk/fields.py

@@ -23,7 +23,7 @@ class MyWrapper(object):
         return self.value == other
 
 
-class MyAutoField(six.with_metaclass(models.SubfieldBase, models.CharField)):
+class MyAutoField(models.CharField):
 
     def __init__(self, *args, **kwargs):
         kwargs['max_length'] = 10
@@ -43,6 +43,11 @@ class MyAutoField(six.with_metaclass(models.SubfieldBase, models.CharField)):
             value = MyWrapper(value)
         return value
 
+    def from_db_value(self, value, connection):
+        if not value:
+            return
+        return MyWrapper(value)
+
     def get_db_prep_save(self, value, connection):
         if not value:
             return

tests/field_subclassing/models.py

@@ -2,13 +2,24 @@
 Tests for field subclassing.
 """
 
+import warnings
+
 from django.db import models
 from django.utils.encoding import force_text
+from django.utils.deprecation import RemovedInDjango20Warning
 
 from .fields import Small, SmallField, SmallerField, JSONField
 from django.utils.encoding import python_2_unicode_compatible
 
 
+# Catch warning about subfieldbase  -- remove in Django 2.0
+warnings.filterwarnings(
+    'ignore',
+    'SubfieldBase has been deprecated. Use Field.from_db_value instead.',
+    RemovedInDjango20Warning
+)
+
+
 @python_2_unicode_compatible
 class MyModel(models.Model):
     name = models.CharField(max_length=10)

tests/from_db_value/models.py

@@ -0,0 +1,32 @@
+import decimal
+
+from django.db import models
+from django.utils.encoding import python_2_unicode_compatible
+
+
+class Cash(decimal.Decimal):
+    currency = 'USD'
+
+    def __str__(self):
+        s = super(Cash, self).__str__(self)
+        return '%s %s' % (s, self.currency)
+
+
+class CashField(models.DecimalField):
+    def __init__(self, **kwargs):
+        kwargs['max_digits'] = 20
+        kwargs['decimal_places'] = 2
+        super(CashField, self).__init__(**kwargs)
+
+    def from_db_value(self, value, connection):
+        cash = Cash(value)
+        cash.vendor = connection.vendor
+        return cash
+
+
+@python_2_unicode_compatible
+class CashModel(models.Model):
+    cash = CashField()
+
+    def __str__(self):
+        return str(self.cash)

tests/from_db_value/tests.py

@@ -0,0 +1,30 @@
+from django.db import connection
+from django.db.models import Max
+from django.test import TestCase
+
+from .models import CashModel, Cash
+
+
+class FromDBValueTest(TestCase):
+    def setUp(self):
+        CashModel.objects.create(cash='12.50')
+
+    def test_simple_load(self):
+        instance = CashModel.objects.get()
+        self.assertIsInstance(instance.cash, Cash)
+
+    def test_values(self):
+        values_list = CashModel.objects.values_list('cash', flat=True)
+        self.assertIsInstance(values_list[0], Cash)
+
+    def test_aggregation(self):
+        maximum = CashModel.objects.aggregate(m=Max('cash'))['m']
+        self.assertIsInstance(maximum, Cash)
+
+    def test_defer(self):
+        instance = CashModel.objects.defer('cash').get()
+        self.assertIsInstance(instance.cash, Cash)
+
+    def test_connection(self):
+        instance = CashModel.objects.get()
+        self.assertEqual(instance.cash.vendor, connection.vendor)

tests/serializers/models.py

@@ -99,7 +99,7 @@ class Team(object):
         return "%s" % self.title
 
 
-class TeamField(six.with_metaclass(models.SubfieldBase, models.CharField)):
+class TeamField(models.CharField):
 
     def __init__(self):
         super(TeamField, self).__init__(max_length=100)
@@ -112,6 +112,9 @@ class TeamField(six.with_metaclass(models.SubfieldBase, models.CharField)):
             return value
         return Team(value)
 
+    def from_db_value(self, value, connection):
+        return Team(value)
+
     def value_to_string(self, obj):
         return self._get_val_from_obj(obj).to_string()