[1.10.x] Refs #27186 -- Fixed model form default fallback for CheckboxSelectMultiple.

Backport of 87c5e7efeb from master

django/forms/widgets.py

@@ -812,6 +812,11 @@ class CheckboxSelectMultiple(RendererMixin, SelectMultiple):
         # require all checkboxes to be checked instead of at least one.
         return False
 
+    def value_omitted_from_data(self, data, files, name):
+        # HTML checkboxes don't appear in POST data if not checked, so it's
+        # never known if the value is actually omitted.
+        return False
+
 
 class MultiWidget(Widget):
     """

docs/ref/forms/widgets.txt

@@ -281,10 +281,11 @@ foundation for custom widgets.
         The method's result affects whether or not a field in a model form
         :ref:`falls back to its default <topics-modelform-save>`.
 
-        A special case is :class:`~django.forms.CheckboxInput`, which always
+        Special cases are :class:`~django.forms.CheckboxInput` and
-        returns ``False`` because an unchecked checkbox doesn't appear in the
+        :class:`~django.forms.CheckboxSelectMultiple`, which always return
-        data of an HTML form submission, so it's unknown whether or not the
+        ``False`` because an unchecked checkbox doesn't appear in the data of
-        user actually submitted a value.
+        an HTML form submission, so it's unknown whether or not the user
+        actually submitted a value.
 
 ``MultiWidget``
 ---------------

docs/releases/1.10.2.txt

@@ -18,10 +18,10 @@ Bugfixes
 * Disabled system check for URL patterns beginning with a '/' when
   ``APPEND_SLASH=False`` (:ticket:`27238`).
 
-* Fixed model form ``default`` fallback for ``MultiWidget``, ``FileInput``,
+* Fixed model form ``default`` fallback for ``CheckboxSelectMultiple``,
-  ``SplitDateTimeWidget``, ``SelectDateWidget``, and ``SplitArrayWidget``
+  ``MultiWidget``, ``FileInput``, ``SplitDateTimeWidget``, ``SelectDateWidget``,
-  (:ticket:`27186`). Custom widgets affected by this issue may need to
+  and ``SplitArrayWidget`` (:ticket:`27186`). Custom widgets affected by this
-  implement a :meth:`~django.forms.Widget.value_omitted_from_data` method.
+  issue should implement :meth:`~django.forms.Widget.value_omitted_from_data`.
 
 * Fixed a crash in ``runserver`` logging during a "Broken pipe" error
   (:ticket:`27271`).

docs/topics/forms/modelforms.txt

@@ -335,7 +335,8 @@ 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 whose
+:class:`~django.forms.CheckboxInput` and
+:class:`~django.forms.CheckboxSelectMultiple` (or any custom widget whose
 :meth:`~django.forms.Widget.value_omitted_from_data` method always returns
 ``False``) 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

tests/forms_tests/widget_tests/test_checkboxselectmultiple.py

@@ -120,3 +120,8 @@ class CheckboxSelectMultipleTest(WidgetTest):
         self.assertIs(widget.use_required_attribute(None), False)
         self.assertIs(widget.use_required_attribute([]), False)
         self.assertIs(widget.use_required_attribute(['J', 'P']), False)
+
+    def test_value_omitted_from_data(self):
+        widget = self.widget(choices=self.beatles)
+        self.assertIs(widget.value_omitted_from_data({}, {}, 'field'), False)
+        self.assertIs(widget.value_omitted_from_data({'field': 'value'}, {}, 'field'), False)

tests/model_forms/tests.py

@@ -585,6 +585,22 @@ class ModelFormBaseTest(TestCase):
         self.assertIsInstance(mf1.fields['active'].widget, forms.CheckboxInput)
         self.assertIs(m1._meta.get_field('active').get_default(), True)
 
+    def test_default_not_populated_on_checkboxselectmultiple(self):
+        class PubForm(forms.ModelForm):
+            mode = forms.CharField(required=False, widget=forms.CheckboxSelectMultiple)
+
+            class Meta:
+                model = PublicationDefaults
+                fields = ('mode',)
+
+        # Empty data doesn't use the model default because an unchecked
+        # CheckboxSelectMultiple doesn't have a value in HTML form submission.
+        mf1 = PubForm({})
+        self.assertEqual(mf1.errors, {})
+        m1 = mf1.save(commit=False)
+        self.assertEqual(m1.mode, '')
+        self.assertEqual(m1._meta.get_field('mode').get_default(), 'di')
+
     def test_prefixed_form_with_default_field(self):
         class PubForm(forms.ModelForm):
             prefix = 'form-prefix'
@@ -635,7 +651,7 @@ class ModelFormBaseTest(TestCase):
         m2 = mf2.save(commit=False)
         self.assertEqual(m2.file.name, 'name')
 
-    def test_selectdatewidget(self):
+    def test_default_selectdatewidget(self):
         class PubForm(forms.ModelForm):
             date_published = forms.DateField(required=False, widget=forms.SelectDateWidget)