Ticket #4102 - Allow UPDATE of only specific fields in model.save() #41

Closed
wants to merge 1 commit into
from
@@ -11,7 +11,7 @@
from django.db.models.fields import AutoField, FieldDoesNotExist
from django.db.models.fields.related import (ManyToOneRel,
OneToOneField, add_lazy_relation)
-from django.db import (connections, router, transaction, DatabaseError,
+from django.db import (router, transaction, DatabaseError,
DEFAULT_DB_ALIAS)
from django.db.models.query import Q
from django.db.models.query_utils import DeferredAttribute
@@ -449,7 +449,8 @@ def serializable_value(self, field_name):
return getattr(self, field_name)
return getattr(self, field.attname)
- def save(self, force_insert=False, force_update=False, using=None):
+ def save(self, force_insert=False, force_update=False, using=None,
+ update_fields=None):
"""
Saves the current instance. Override this in a subclass if you want to
control the saving process.
@@ -458,22 +459,41 @@ def save(self, force_insert=False, force_update=False, using=None):
that the "save" must be an SQL insert or update (or equivalent for
non-SQL backends), respectively. Normally, they should not be set.
"""
- if force_insert and force_update:
+ if force_insert and (force_update or update_fields):
raise ValueError("Cannot force both insert and updating in model saving.")
- self.save_base(using=using, force_insert=force_insert, force_update=force_update)
+ if update_fields is not None:
+ # If update_fields is empty, skip the save. We do also check for
+ # no-op saves later on for inheritance cases. This bailout is
+ # still needed for skipping signal sending.
+ if len(update_fields) == 0:
+ return
+
+ update_fields = frozenset(update_fields)
+ field_names = set([field.name for field in self._meta.fields
+ if not field.primary_key])
+ non_model_fields = update_fields.difference(field_names)
+
+ if non_model_fields:
+ raise ValueError("The following fields do not exist in this "
+ "model or are m2m fields: %s"
+ % ', '.join(non_model_fields))
+
+ self.save_base(using=using, force_insert=force_insert,
+ force_update=force_update, update_fields=update_fields)
save.alters_data = True
def save_base(self, raw=False, cls=None, origin=None, force_insert=False,
- force_update=False, using=None):
+ force_update=False, using=None, update_fields=None):
"""
Does the heavy-lifting involved in saving. Subclasses shouldn't need to
override this method. It's separate from save() in order to hide the
need for overrides of save() to pass around internal-only parameters
('raw', 'cls', and 'origin').
"""
using = using or router.db_for_write(self.__class__, instance=self)
- assert not (force_insert and force_update)
+ assert not (force_insert and (force_update or update_fields))
+ assert update_fields is None or len(update_fields) > 0
if cls is None:
cls = self.__class__
meta = cls._meta
@@ -483,7 +503,8 @@ def save_base(self, raw=False, cls=None, origin=None, force_insert=False,
meta = cls._meta
if origin and not meta.auto_created:
- signals.pre_save.send(sender=origin, instance=self, raw=raw, using=using)
+ signals.pre_save.send(sender=origin, instance=self, raw=raw, using=using,
+ update_fields=update_fields)
# If we are in a raw save, save the object exactly as presented.
# That means that we don't try to be smart about saving attributes
@@ -503,7 +524,8 @@ def save_base(self, raw=False, cls=None, origin=None, force_insert=False,
if field and getattr(self, parent._meta.pk.attname) is None and getattr(self, field.attname) is not None:
setattr(self, parent._meta.pk.attname, getattr(self, field.attname))
- self.save_base(cls=parent, origin=org, using=using)
+ self.save_base(cls=parent, origin=org, using=using,
+ update_fields=update_fields)
if field:
setattr(self, field.attname, self._get_pk_val(parent._meta))
@@ -513,22 +535,27 @@ def save_base(self, raw=False, cls=None, origin=None, force_insert=False,
if not meta.proxy:
non_pks = [f for f in meta.local_fields if not f.primary_key]
+ if update_fields:
+ non_pks = [f for f in non_pks if f.name in update_fields]
+
# First, try an UPDATE. If that doesn't update anything, do an INSERT.
pk_val = self._get_pk_val(meta)
pk_set = pk_val is not None
record_exists = True
manager = cls._base_manager
if pk_set:
- # Determine whether a record with the primary key already exists.
- if (force_update or (not force_insert and
+ # Determine if we should do an update (pk already exists, forced update,
+ # no force_insert)
+ if ((force_update or update_fields) or (not force_insert and
manager.using(using).filter(pk=pk_val).exists())):
- # It does already exist, so do an UPDATE.
if force_update or non_pks:
values = [(f, None, (raw and getattr(self, f.attname) or f.pre_save(self, False))) for f in non_pks]
if values:
rows = manager.using(using).filter(pk=pk_val)._update(values)
if force_update and not rows:
raise DatabaseError("Forced update did not affect any rows.")
+ if update_fields and not rows:
+ raise DatabaseError("Save with update_fields did not affect any rows.")
else:
record_exists = False
if not pk_set or not record_exists:
@@ -541,7 +568,7 @@ def save_base(self, raw=False, cls=None, origin=None, force_insert=False,
fields = meta.local_fields
if not pk_set:
- if force_update:
+ if force_update or update_fields:
raise ValueError("Cannot force an update in save() with no primary key.")
fields = [f for f in fields if not isinstance(f, AutoField)]
@@ -561,8 +588,8 @@ def save_base(self, raw=False, cls=None, origin=None, force_insert=False,
# Signal that the save is complete
if origin and not meta.auto_created:
- signals.post_save.send(sender=origin, instance=self,
- created=(not record_exists), raw=raw, using=using)
+ signals.post_save.send(sender=origin, instance=self, created=(not record_exists),
+ update_fields=update_fields, raw=raw, using=using)
save_base.alters_data = True
@@ -5,8 +5,8 @@
pre_init = Signal(providing_args=["instance", "args", "kwargs"])
post_init = Signal(providing_args=["instance"])
-pre_save = Signal(providing_args=["instance", "raw", "using"])
-post_save = Signal(providing_args=["instance", "raw", "created", "using"])
+pre_save = Signal(providing_args=["instance", "raw", "using", "update_fields"])
+post_save = Signal(providing_args=["instance", "raw", "created", "using", "update_fields"])
pre_delete = Signal(providing_args=["instance", "using"])
post_delete = Signal(providing_args=["instance", "using"])
@@ -135,7 +135,7 @@ Saving objects
To save an object back to the database, call ``save()``:
-.. method:: Model.save([force_insert=False, force_update=False, using=DEFAULT_DB_ALIAS])
+.. method:: Model.save([force_insert=False, force_update=False, using=DEFAULT_DB_ALIAS, update_fields=None])
.. versionadded:: 1.2
The ``using`` argument was added.
@@ -289,6 +289,8 @@ almost always do the right thing and trying to override that will lead to
errors that are difficult to track down. This feature is for advanced use
only.
+Using ``update_fields`` will force an update similarly to ``force_update``.
+
Updating attributes based on existing fields
--------------------------------------------
@@ -334,6 +336,24 @@ For more details, see the documentation on :ref:`F() expressions
<query-expressions>` and their :ref:`use in update queries
<topics-db-queries-update>`.
+Specifying which fields to save
+-------------------------------
+
+.. versionadded:: 1.5
+
+If ``save()`` is passed a list of field names in keyword argument
+``update_fields``, only the fields named in that list will be updated.
+This may be desirable if you want to update just one or a few fields on
+an object. There will be a slight performance benefit from preventing
+all of the model fields from being updated in the database. For example:
+
+ product.name = 'Name changed again'
+ product.save(update_fields=['name'])
+
+The ``update_fields`` argument can be any iterable containing strings. An
@freakboy3742
freakboy3742 May 12, 2012 Django member

"An not None" should be "A non-None"; but the whole sentence is a bit confusing anyway. Consider rewording the whole sentence. "An empty update_fields iterable will skip the save. A value of None will perform an update on all fields".

+empty ``update_fields`` iterable will skip the save. A value of None will
+perform an update on all fields.
+
Deleting objects
================
@@ -123,6 +123,10 @@ Arguments sent with this signal:
``using``
The database alias being used.
+``update_fields``
+ A frozenset of fields to update explicitly specified in the ``save()`` method.
+ ``None`` if you do not use this parameter when you call ``save()``.
+
post_save
---------
@@ -154,6 +158,10 @@ Arguments sent with this signal:
``using``
The database alias being used.
+``update_fields``
+ A frozenset of fields to update explicitly specified in the ``save()`` method.
+ ``None`` if you do not use this parameter when you call ``save()``.
+
pre_delete
----------
View
@@ -33,6 +33,17 @@ version compatible with Python 2.6.
What's new in Django 1.5
========================
+Support for saving a subset of model's fields
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+The method :meth:`Model.save() <django.db.models.Model.save()>` has a new
+keyword argument ``update_fields``. By using this argument it is possible to
+save only a select list of model's fields. This can be useful for performance
+reasons or when trying to avoid overwriting concurrent changes.
+
+See the :meth:`Model.save() <django.db.models.Model.save()>` documentation for
+more details.
+
Minor features
~~~~~~~~~~~~~~
@@ -0,0 +1,37 @@
+
+from django.db import models
+
+GENDER_CHOICES = (
+ ('M', 'Male'),
+ ('F', 'Female'),
+)
+
+class Account(models.Model):
+ num = models.IntegerField()
+
+
+class Person(models.Model):
+ name = models.CharField(max_length=20)
+ gender = models.CharField(max_length=1, choices=GENDER_CHOICES)
+
+ def __unicode__(self):
+ return self.name
+
+
+class Employee(Person):
+ employee_num = models.IntegerField(default=0)
+ profile = models.ForeignKey('Profile', related_name='profiles', null=True)
+ accounts = models.ManyToManyField('Account', related_name='employees', blank=True, null=True)
+
+
+class Profile(models.Model):
+ name = models.CharField(max_length=200)
+ salary = models.FloatField(default=1000.0)
+
+ def __unicode__(self):
+ return self.name
+
+
+class ProxyEmployee(Employee):
+ class Meta:
+ proxy = True
Oops, something went wrong. Retry.