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

Backport of 4bc6b93994 from master

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()