Fixed #24245 -- Added introspection for database defaults.

Needed for tests for migrations handling of database defaults.

django/db/backends/base/features.py

@@ -125,6 +125,9 @@ class BaseDatabaseFeatures(object):
     # This is True for all core backends.
     can_introspect_null = True
 
+    # Can the backend introspect the default value of a column?
+    can_introspect_default = True
+
     # Confirm support for introspected foreign keys
     # Every database can do this reliably, except MySQL,
     # which can't do it for MyISAM tables

django/db/backends/mysql/introspection.py

@@ -9,8 +9,8 @@ from django.utils.encoding import force_text
 
 from MySQLdb.constants import FIELD_TYPE
 
-FieldInfo = namedtuple('FieldInfo', FieldInfo._fields + ('extra',))
-
+FieldInfo = namedtuple('FieldInfo', FieldInfo._fields + ('extra', 'default'))
+InfoLine = namedtuple('InfoLine', 'col_name data_type max_len num_prec num_scale extra column_default')
 foreign_key_re = re.compile(r"\sCONSTRAINT `[^`]*` FOREIGN KEY \(`([^`]*)`\) REFERENCES `([^`]*)` \(`([^`]*)`\)")
 
 
@@ -61,9 +61,9 @@ class DatabaseIntrospection(BaseDatabaseIntrospection):
         #   not visible length (#5725)
         # - precision and scale (for decimal fields) (#5014)
         # - auto_increment is not available in cursor.description
-        InfoLine = namedtuple('InfoLine', 'col_name data_type max_len num_prec num_scale extra')
         cursor.execute("""
-            SELECT column_name, data_type, character_maximum_length, numeric_precision, numeric_scale, extra
+            SELECT column_name, data_type, character_maximum_length, numeric_precision,
+                   numeric_scale, extra, column_default
             FROM information_schema.columns
             WHERE table_name = %s AND table_schema = DATABASE()""", [table_name])
         field_info = {line[0]: InfoLine(*line) for line in cursor.fetchall()}
@@ -80,7 +80,8 @@ class DatabaseIntrospection(BaseDatabaseIntrospection):
                                to_int(field_info[col_name].num_prec) or line[4],
                                to_int(field_info[col_name].num_scale) or line[5])
                             + (line[6],)
-                            + (field_info[col_name].extra,)))
+                            + (field_info[col_name].extra,)
+                            + (field_info[col_name].column_default,)))
             )
         return fields
 

django/db/backends/oracle/features.py

@@ -28,6 +28,7 @@ class DatabaseFeatures(BaseDatabaseFeatures):
     has_bulk_insert = True
     supports_tablespaces = True
     supports_sequence_reset = False
+    can_introspect_default = False  # Pending implementation by an interested person.
     can_introspect_max_length = False
     can_introspect_time_field = False
     atomic_transactions = False

django/db/backends/sqlite3/introspection.py

@@ -1,4 +1,5 @@
 import re
+from collections import namedtuple
 
 from django.db.backends.base.introspection import (
     BaseDatabaseIntrospection, FieldInfo, TableInfo,
@@ -6,6 +7,7 @@ from django.db.backends.base.introspection import (
 
 
 field_size_re = re.compile(r'^\s*(?:var)?char\s*\(\s*(\d+)\s*\)\s*$')
+FieldInfo = namedtuple('FieldInfo', FieldInfo._fields + ('default',))
 
 
 def get_field_size(name):
@@ -69,8 +71,18 @@ class DatabaseIntrospection(BaseDatabaseIntrospection):
 
     def get_table_description(self, cursor, table_name):
         "Returns a description of the table, with the DB-API cursor.description interface."
-        return [FieldInfo(info['name'], info['type'], None, info['size'], None, None,
-                 info['null_ok']) for info in self._table_info(cursor, table_name)]
+        return [
+            FieldInfo(
+                info['name'],
+                info['type'],
+                None,
+                info['size'],
+                None,
+                None,
+                info['null_ok'],
+                info['default'],
+            ) for info in self._table_info(cursor, table_name)
+        ]
 
     def column_name_converter(self, name):
         """
@@ -211,13 +223,15 @@ class DatabaseIntrospection(BaseDatabaseIntrospection):
 
     def _table_info(self, cursor, name):
         cursor.execute('PRAGMA table_info(%s)' % self.connection.ops.quote_name(name))
-        # cid, name, type, notnull, dflt_value, pk
-        return [{'name': field[1],
-                 'type': field[2],
-                 'size': get_field_size(field[2]),
-                 'null_ok': not field[3],
-                 'pk': field[5]     # undocumented
-                 } for field in cursor.fetchall()]
+        # cid, name, type, notnull, default_value, pk
+        return [{
+            'name': field[1],
+            'type': field[2],
+            'size': get_field_size(field[2]),
+            'null_ok': not field[3],
+            'default': field[4],
+            'pk': field[5],  # undocumented
+        } for field in cursor.fetchall()]
 
     def get_constraints(self, cursor, table_name):
         """

docs/releases/1.9.txt

@@ -165,6 +165,15 @@ Backwards incompatible changes in 1.9
     deprecation timeline for a given feature, its removal may appear as a
     backwards incompatible change.
 
+Database backend API
+~~~~~~~~~~~~~~~~~~~~
+
+* A couple of new tests rely on the ability of the backend to introspect column
+  defaults (returning the result as ``Field.default``). You can set the
+  ``can_introspect_default`` database feature to ``False`` if your backend
+  doesn't implement this. You may want to review the implementation on the
+  backends that Django includes for reference (:ticket:`24245`).
+
 Miscellaneous
 ~~~~~~~~~~~~~
 

tests/schema/tests.py

@@ -1348,3 +1348,53 @@ class SchemaTests(TransactionTestCase):
         finally:
             # Cleanup model states
             AuthorWithM2M._meta.local_many_to_many.remove(new_field)
+
+    def test_add_field_default_dropped(self):
+        # Create the table
+        with connection.schema_editor() as editor:
+            editor.create_model(Author)
+        # Ensure there's no surname field
+        columns = self.column_classes(Author)
+        self.assertNotIn("surname", columns)
+        # Create a row
+        Author.objects.create(name='Anonymous1')
+        # Add new CharField with a default
+        new_field = CharField(max_length=15, blank=True, default='surname default')
+        new_field.set_attributes_from_name("surname")
+        with connection.schema_editor() as editor:
+            editor.add_field(Author, new_field)
+        # Ensure field was added with the right default
+        with connection.cursor() as cursor:
+            cursor.execute("SELECT surname FROM schema_author;")
+            item = cursor.fetchall()[0]
+            self.assertEqual(item[0], 'surname default')
+            # And that the default is no longer set in the database.
+            field = next(
+                f for f in connection.introspection.get_table_description(cursor, "schema_author")
+                if f.name == "surname"
+            )
+            if connection.features.can_introspect_default:
+                self.assertIsNone(field.default)
+
+    def test_alter_field_default_dropped(self):
+        # Create the table
+        with connection.schema_editor() as editor:
+            editor.create_model(Author)
+        # Create a row
+        Author.objects.create(name='Anonymous1')
+        self.assertEqual(Author.objects.get().height, None)
+        old_field = Author._meta.get_field('height')
+        # The default from the new field is used in updating existing rows.
+        new_field = IntegerField(blank=True, default=42)
+        new_field.set_attributes_from_name('height')
+        with connection.schema_editor() as editor:
+            editor.alter_field(Author, old_field, new_field)
+        self.assertEqual(Author.objects.get().height, 42)
+        # The database default should be removed.
+        with connection.cursor() as cursor:
+            field = next(
+                f for f in connection.introspection.get_table_description(cursor, "schema_author")
+                if f.name == "height"
+            )
+            if connection.features.can_introspect_default:
+                self.assertIsNone(field.default)