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Fixed #19501 -- added Model.from_db() method 

The Model.from_db() is intended to be used in cases where customization
of model loading is needed. Reasons can be performance, or adding custom
behavior to the model (for example "dirty field tracking" to issue
automatic update_fields when saving models).

A big thank you to Tim Graham for the review!
This commit is contained in:
Anssi Kääriäinen 2014-06-06 15:35:40 +03:00
parent c26579eaa7
commit 0b6f05ede6
4 changed files with 119 additions and 64 deletions
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10
django/db/models/base.py
View File

@ -458,6 +458,16 @@ class Model(six.with_metaclass(ModelBase)):
super(Model, self).__init__()
signals.post_init.send(sender=self.__class__, instance=self)
@classmethod
def from_db(cls, db, field_names, values):
if cls._deferred:
new = cls(**dict(zip(field_names, values)))
else:
new = cls(*values)
new._state.adding = False
new._state.db = db
return new
def __repr__(self):
try:
u = six.text_type(self)

115
django/db/models/query.py
View File

@ -241,8 +241,7 @@ class QuerySet(object):
aggregate_select = list(self.query.aggregate_select)
only_load = self.query.get_loaded_field_names()
if not fill_cache:
fields = self.model._meta.concrete_fields
fields = self.model._meta.concrete_fields
load_fields = []
# If only/defer clauses have been specified,
@ -260,9 +259,6 @@ class QuerySet(object):
# Therefore, we need to load all fields from this model
load_fields.append(field.name)
index_start = len(extra_select)
aggregate_start = index_start + len(load_fields or self.model._meta.concrete_fields)
skip = None
if load_fields and not fill_cache:
# Some fields have been deferred, so we have to initialize
@ -275,30 +271,25 @@ class QuerySet(object):
else:
init_list.append(field.attname)
model_cls = deferred_class_factory(self.model, skip)
else:
model_cls = self.model
init_list = [f.attname for f in fields]
# Cache db and model outside the loop
db = self.db
model = self.model
compiler = self.query.get_compiler(using=db)
index_start = len(extra_select)
aggregate_start = index_start + len(init_list)
if fill_cache:
klass_info = get_klass_info(model, max_depth=max_depth,
klass_info = get_klass_info(model_cls, max_depth=max_depth,
requested=requested, only_load=only_load)
for row in compiler.results_iter():
if fill_cache:
obj, _ = get_cached_row(row, index_start, db, klass_info,
offset=len(aggregate_select))
else:
# Omit aggregates in object creation.
row_data = row[index_start:aggregate_start]
if skip:
obj = model_cls(**dict(zip(init_list, row_data)))
else:
obj = model(*row_data)
# Store the source database of the object
obj._state.db = db
# This object came from the database; it's not being added.
obj._state.adding = False
obj = model_cls.from_db(db, init_list, row[index_start:aggregate_start])
if extra_select:
for i, k in enumerate(extra_select):
@ -1417,6 +1408,21 @@ def get_klass_info(klass, max_depth=0, cur_depth=0, requested=None,
return klass, field_names, field_count, related_fields, reverse_related_fields, pk_idx
def reorder_for_init(model, field_names, values):
"""
Reorders given field names and values for those fields
to be in the same order as model.__init__() expects to find them.
"""
new_names, new_values = [], []
for f in model._meta.concrete_fields:
if f.attname not in field_names:
continue
new_names.append(f.attname)
new_values.append(values[field_names.index(f.attname)])
assert len(new_names) == len(field_names)
return new_names, new_values
def get_cached_row(row, index_start, using, klass_info, offset=0,
parent_data=()):
"""
@ -1451,18 +1457,19 @@ def get_cached_row(row, index_start, using, klass_info, offset=0,
fields[pk_idx] == '')):
obj = None
elif field_names:
fields = list(fields)
values = list(fields)
parent_values = []
parent_field_names = []
for rel_field, value in parent_data:
field_names.append(rel_field.attname)
fields.append(value)
obj = klass(**dict(zip(field_names, fields)))
parent_field_names.append(rel_field.attname)
parent_values.append(value)
field_names, values = reorder_for_init(
klass, parent_field_names + field_names,
parent_values + values)
obj = klass.from_db(using, field_names, values)
else:
obj = klass(*fields)
# If an object was retrieved, set the database state.
if obj:
obj._state.db = using
obj._state.adding = False
field_names = [f.attname for f in klass._meta.concrete_fields]
obj = klass.from_db(using, field_names, fields)
# Instantiate related fields
index_end = index_start + field_count + offset
# Iterate over each related object, populating any
@ -1494,7 +1501,7 @@ def get_cached_row(row, index_start, using, klass_info, offset=0,
parent_data.append((rel_field, getattr(obj, rel_field.attname)))
# Recursively retrieve the data for the related object
cached_row = get_cached_row(row, index_end, using, klass_info,
parent_data=parent_data)
parent_data=parent_data)
# If the recursive descent found an object, populate the
# descriptor caches relevant to the object
if cached_row:
@ -1534,15 +1541,18 @@ class RawQuerySet(object):
self.params = params or ()
self.translations = translations or {}
def __iter__(self):
# Mapping of attrnames to row column positions. Used for constructing
# the model using kwargs, needed when not all model's fields are present
# in the query.
model_init_field_names = {}
# A list of tuples of (column name, column position). Used for
# annotation fields.
annotation_fields = []
def resolve_model_init_order(self):
"""
Resolve the init field names and value positions
"""
model_init_names = [f.attname for f in self.model._meta.fields
if f.attname in self.columns]
annotation_fields = [(column, pos) for pos, column in enumerate(self.columns)
if column not in self.model_fields]
model_init_order = [self.columns.index(fname) for fname in model_init_names]
return model_init_names, model_init_order, annotation_fields
def __iter__(self):
# Cache some things for performance reasons outside the loop.
db = self.db
compiler = connections[db].ops.compiler('SQLCompiler')(
@ -1553,18 +1563,12 @@ class RawQuerySet(object):
query = iter(self.query)
try:
# Find out which columns are model's fields, and which ones should be
# annotated to the model.
for pos, column in enumerate(self.columns):
if column in self.model_fields:
model_init_field_names[self.model_fields[column].attname] = pos
else:
annotation_fields.append((column, pos))
model_init_names, model_init_pos, annotation_fields = self.resolve_model_init_order()
# Find out which model's fields are not present in the query.
skip = set()
for field in self.model._meta.fields:
if field.attname not in model_init_field_names:
if field.attname not in model_init_names:
skip.add(field.attname)
if skip:
if self.model._meta.pk.attname in skip:
@ -1572,34 +1576,17 @@ class RawQuerySet(object):
model_cls = deferred_class_factory(self.model, skip)
else:
model_cls = self.model
# All model's fields are present in the query. So, it is possible
# to use *args based model instantiation. For each field of the model,
# record the query column position matching that field.
model_init_field_pos = []
for field in self.model._meta.fields:
model_init_field_pos.append(model_init_field_names[field.attname])
if need_resolv_columns:
fields = [self.model_fields.get(c, None) for c in self.columns]
# Begin looping through the query values.
for values in query:
if need_resolv_columns:
values = compiler.resolve_columns(values, fields)
# Associate fields to values
if skip:
model_init_kwargs = {}
for attname, pos in six.iteritems(model_init_field_names):
model_init_kwargs[attname] = values[pos]
instance = model_cls(**model_init_kwargs)
else:
model_init_args = [values[pos] for pos in model_init_field_pos]
instance = model_cls(*model_init_args)
model_init_values = [values[pos] for pos in model_init_pos]
instance = model_cls.from_db(db, model_init_names, model_init_values)
if annotation_fields:
for column, pos in annotation_fields:
setattr(instance, column, values[pos])
instance._state.db = db
instance._state.adding = False
yield instance
finally:
# Done iterating the Query. If it has its own cursor, close it.

54
docs/ref/models/instances.txt
View File

@ -62,6 +62,60 @@ that, you need to :meth:`~Model.save()`.
book = Book.objects.create_book("Pride and Prejudice")
Customizing model loading
-------------------------
.. classmethod:: Model.from_db(db, field_names, values)
.. versionadded:: 1.8
The ``from_db()`` method can be used to customize model instance creation
when loading from the database.
The ``db`` argument contains the database alias for the database the model
is loaded from, ``field_names`` contains the names of all loaded fields, and
``values`` contains the loaded values for each field in ``field_names``. The
``field_names`` are in the same order as the ``values``, so it is possible to
use ``cls(**(zip(field_names, values)))`` to instantiate the object. If all
of the model's fields are present, then ``values`` are guaranteed to be in
the order ``__init__()`` expects them. That is, the instance can be created
by ``cls(*values)``. It is possible to check if all fields are present by
consulting ``cls._deferred`` - if ``False``, then all fields have been loaded
from the database.
In addition to creating the new model, the ``from_db()`` method must set the
``adding`` and ``db`` flags in the new instance's ``_state`` attribute.
Below is an example showing how torecord the initial values of fields that
are loaded from the database::
@classmethod
def from_db(cls, db, field_names, values):
# default implementation of from_db() (could be replaced
# with super())
if cls._deferred:
instance = cls(**zip(field_names, values))
else:
instance = cls(*values)
instance._state.adding = False
instance._state.db = db
# customization to store the original field values on the instance
instance._loaded_values = zip(field_names, values)
return instance
def save(self, *args, **kwargs):
# Check how the current values differ from ._loaded_values. For example,
# prevent changing the creator_id of the model. (This example doesn't
# support cases where 'creator_id' is deferred).
if not self._state.adding and (
self.creator_id != self._loaded_values['creator_id']):
raise ValueError("Updating the value of creator isn't allowed")
super(...).save(*args, **kwargs)
The example above shows a full ``from_db()`` implementation to clarify how that
is done. In this case it would of course be possible to just use ``super()`` call
in the ``from_db()`` method.
.. _validating-objects:
Validating objects

4
docs/releases/1.8.txt
View File

@ -193,6 +193,10 @@ Models
when these objects are unpickled in a different version than the one in
which they were pickled.
* Added :meth:`Model.from_db() <django.db.models.Model.from_db()>` which
Django uses whenever objects are loaded using the ORM. The method allows
customizing model loading behavior.
Signals
^^^^^^^