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: http://code.djangoproject.com/svn/django/trunk@5804 bcc190cf-cafb-0310-a4f2-bffc1f526a37

django/newforms/models.py

@@ -35,17 +35,23 @@ def save_instance(form, instance, fields=None, fail_message='saved', commit=True
         if fields and f.name not in fields:
             continue
         setattr(instance, f.name, cleaned_data[f.name])
-    if commit:
-        instance.save()
+    # 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 f.name not in fields:
                 continue
             if f.name in cleaned_data:
                 setattr(instance, f.attname, cleaned_data[f.name])
-    # 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
+        instance.save()
+        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):

docs/newforms.txt

@@ -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 = f.save(commit=False)
+
+    # Modify the author in some way
+    ...
+    # Save the new instance
+    >>> new_author.save()
+
+    # 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
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 

tests/modeltests/model_forms/models.py

@@ -332,6 +332,28 @@ Create a new article, with no categories, via the form.
 >>> 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 = f.save(commit=False)
+
+# Manually save the instance 
+>>> new_art.save()
+>>> new_art.id
+4
+
+# 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.