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Fixed #4001 -- Added dynamic save_m2m method() to forms created with…

… form_for_model and form_for_instance on save(commit=False).

git-svn-id: bcc190cf-cafb-0310-a4f2-bffc1f526a37
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freakboy3742 committed Aug 5, 2007
1 parent 212ee65 commit f96e933534501fd98f84cc53bcac62beaa72dbf2
Showing with 64 additions and 6 deletions.
  1. +12 −6 django/newforms/
  2. +30 −0 docs/newforms.txt
  3. +22 −0 tests/modeltests/model_forms/
@@ -35,17 +35,23 @@ def save_instance(form, instance, fields=None, fail_message='saved', commit=True
if fields and not in fields:
setattr(instance,, cleaned_data[])
- if commit:
+ # Wrap up the saving of m2m data as a function
+ def save_m2m():
+ opts = instance.__class__._meta
+ cleaned_data = form.cleaned_data
for f in opts.many_to_many:
if fields and not in fields:
if in cleaned_data:
setattr(instance, f.attname, cleaned_data[])
- # GOTCHA: If many-to-many data is given and commit=False, the many-to-many
- # data will be lost. This happens because a many-to-many options cannot be
- # set on an object until after it's saved. Maybe we should raise an
- # exception in that case.
+ if commit:
+ # If we are committing, save the instance and the m2m data immediately
+ save_m2m()
+ else:
+ # We're not committing. Add a method to the form to allow deferred
+ # saving of m2m data
+ form.save_m2m = save_m2m
return instance
def make_model_save(model, fields, fail_message):
@@ -1502,6 +1502,36 @@ the database. In this case, it's up to you to call ``save()`` on the resulting
model instance. This is useful if you want to do custom processing on the
object before saving it. ``commit`` is ``True`` by default.
+Another side effect of using ``commit=False`` is seen when your model has
+a many-to-many relation with another model. If your model has a many-to-many
+relation and you specify ``commit=False`` when you save a form, Django cannot
+immediately save the form data for the many-to-many relation. This is because
+it isn't possible to save many-to-many data for an instance until the instance
+exists in the database.
+To work around this problem, every time you save a form using ``commit=False``,
+Django adds a ``save_m2m()`` method to the form created by ``form_for_model``.
+After you have manually saved the instance produced by the form, you can invoke
+``save_m2m()`` to save the many-to-many form data::
+ # Create a form instance with POST data.
+ >>> f = AuthorForm(request.POST)
+ # Create, but don't save the new author instance
+ >>> new_author =
+ # Modify the author in some way
+ ...
+ # Save the new instance
+ >>>
+ # Now save the many-to-many data for the form
+ >>> f.save_m2m()
+Calling ``save_m2m()`` is only required if you use ``save(commit=False)``.
+When you use a simple ``save()`` on a form, all data - include
+many-to-many data - is saved without the need for any additional method calls.
Using an alternate base class
@@ -332,6 +332,28 @@ def __unicode__(self):
>>> new_art.categories.all()
+Create a new article, with categories, via the form, but use commit=False.
+The m2m data won't be saved until save_m2m() is invoked on the form.
+>>> ArticleForm = form_for_model(Article)
+>>> f = ArticleForm({'headline': u'The walrus was Paul', 'pub_date': u'1967-11-01',
+... 'writer': u'1', 'article': u'Test.', 'categories': [u'1', u'2']})
+>>> new_art =
+# Manually save the instance
+# The instance doesn't have m2m data yet
+>>> new_art = Article.objects.get(id=4)
+>>> new_art.categories.all()
+# Save the m2m data on the form
+>>> f.save_m2m()
+>>> new_art.categories.all()
+[<Category: Entertainment>, <Category: It's a test>]
Here, we define a custom Form. Because it happens to have the same fields as
the Category model, we can use save_instance() to apply its changes to an
existing Category instance.

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