[1.10.x] Fixed #27039 -- Fixed empty data fallback to model field default in model forms.

Backport of 4bc6b93994 from master

django/forms/models.py

@@ -52,6 +52,11 @@ def construct_instance(form, instance, fields=None, exclude=None):
             continue
         if exclude and f.name in exclude:
             continue
+        # Leave defaults for fields that aren't in POST data, except for
+        # checkbox inputs because they don't appear in POST data if not checked.
+        if (f.has_default() and f.name not in form.data and
+                not getattr(form[f.name].field.widget, 'dont_use_model_field_default_for_empty_data', False)):
+            continue
         # Defer saving file-type fields until after the other fields, so a
         # callable upload_to can use the values from other fields.
         if isinstance(f, models.FileField):

django/forms/widgets.py

@@ -480,6 +480,10 @@ def boolean_check(v):
 
 
 class CheckboxInput(Widget):
+    # Don't use model field defaults for fields that aren't in POST data,
+    # because checkboxes don't appear in POST data if not checked.
+    dont_use_model_field_default_for_empty_data = True
+
     def __init__(self, attrs=None, check_test=None):
         super(CheckboxInput, self).__init__(attrs)
         # check_test is a callable that takes a value and returns True

docs/releases/1.10.1.txt

@@ -79,3 +79,6 @@ Bugfixes
 
 * Reallowed subclassing ``UserCreationForm`` without ``USERNAME_FIELD`` in
   ``Meta.fields`` (:ticket:`27111`).
+
+* Fixed a regression in model forms where model fields with a ``default`` that
+  didn't appear in POST data no longer used the ``default`` (:ticket:`27039`).

docs/topics/forms/modelforms.txt

@@ -330,6 +330,21 @@ Note that if the form :ref:`hasn't been validated
 ``form.errors``. A ``ValueError`` will be raised if the data in the form
 doesn't validate -- i.e., if ``form.errors`` evaluates to ``True``.
 
+If an optional field doesn't appear in the form's data, the resulting model
+instance uses the model field :attr:`~django.db.models.Field.default`, if
+there is one, for that field. This behavior doesn't apply to fields that use
+:class:`~django.forms.CheckboxInput` (or any custom widget with
+``dont_use_model_field_default_for_empty_data=True``) since an unchecked
+checkbox doesn't appear in the data of an HTML form submission. Use a custom
+form field or widget if you're designing an API and want the default fallback
+for a ``BooleanField``.
+
+.. versionchanged:: 1.10.1
+
+    Older versions don't have the exception for
+    :class:`~django.forms.CheckboxInput` which means that unchecked checkboxes
+    receive a value of ``True`` if that's the model field default.
+
 This ``save()`` method accepts an optional ``commit`` keyword argument, which
 accepts either ``True`` or ``False``. If you call ``save()`` with
 ``commit=False``, then it will return an object that hasn't yet been saved to

tests/model_forms/models.py

@@ -122,6 +122,7 @@ class PublicationDefaults(models.Model):
     date_published = models.DateField(default=datetime.date.today)
     mode = models.CharField(max_length=2, choices=MODE_CHOICES, default=default_mode)
     category = models.IntegerField(choices=CATEGORY_CHOICES, default=default_category)
+    active = models.BooleanField(default=True)
 
 
 class Author(models.Model):

tests/model_forms/tests.py

@@ -549,6 +549,42 @@ class ModelFormBaseTest(TestCase):
         self.assertEqual(list(OrderFields2.base_fields),
                          ['slug', 'name'])
 
+    def test_default_populated_on_optional_field(self):
+        class PubForm(forms.ModelForm):
+            mode = forms.CharField(max_length=255, required=False)
+
+            class Meta:
+                model = PublicationDefaults
+                fields = ('mode',)
+
+        # Empty data uses the model field default.
+        mf1 = PubForm({})
+        self.assertEqual(mf1.errors, {})
+        m1 = mf1.save(commit=False)
+        self.assertEqual(m1.mode, 'di')
+        self.assertEqual(m1._meta.get_field('mode').get_default(), 'di')
+
+        # Blank data doesn't use the model field default.
+        mf2 = PubForm({'mode': ''})
+        self.assertEqual(mf2.errors, {})
+        m2 = mf2.save(commit=False)
+        self.assertEqual(m2.mode, '')
+
+    def test_default_not_populated_on_optional_checkbox_input(self):
+        class PubForm(forms.ModelForm):
+            class Meta:
+                model = PublicationDefaults
+                fields = ('active',)
+
+        # Empty data doesn't use the model default because CheckboxInput
+        # doesn't have a value in HTML form submission.
+        mf1 = PubForm({})
+        self.assertEqual(mf1.errors, {})
+        m1 = mf1.save(commit=False)
+        self.assertIs(m1.active, False)
+        self.assertIsInstance(mf1.fields['active'].widget, forms.CheckboxInput)
+        self.assertIs(m1._meta.get_field('active').get_default(), True)
+
 
 class FieldOverridesByFormMetaForm(forms.ModelForm):
     class Meta:
@@ -758,7 +794,10 @@ class UniqueTest(TestCase):
         title = 'Boss'
         isbn = '12345'
         DerivedBook.objects.create(title=title, author=self.writer, isbn=isbn)
-        form = DerivedBookForm({'title': 'Other', 'author': self.writer.pk, 'isbn': isbn})
+        form = DerivedBookForm({
+            'title': 'Other', 'author': self.writer.pk, 'isbn': isbn,
+            'suffix1': '1', 'suffix2': '2',
+        })
         self.assertFalse(form.is_valid())
         self.assertEqual(len(form.errors), 1)
         self.assertEqual(form.errors['isbn'], ['Derived book with this Isbn already exists.'])
@@ -2443,7 +2482,7 @@ class OtherModelFormTests(TestCase):
         class PublicationDefaultsForm(forms.ModelForm):
             class Meta:
                 model = PublicationDefaults
-                fields = '__all__'
+                fields = ('title', 'date_published', 'mode', 'category')
 
         self.maxDiff = 2000
         form = PublicationDefaultsForm()