[1.10.x] Refs #27039 -- Fixed regression with field defaults in prefixed forms.

Backport of d9c083cfee from master

django/forms/models.py

@@ -54,7 +54,7 @@ def construct_instance(form, instance, fields=None, exclude=None):
             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
+        if (f.has_default() and form.add_prefix(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

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_prefixed_form_with_default_field(self):
+        class PubForm(forms.ModelForm):
+            prefix = 'form-prefix'
+
+            class Meta:
+                model = PublicationDefaults
+                fields = ('mode',)
+
+        mode = 'de'
+        self.assertNotEqual(mode, PublicationDefaults._meta.get_field('mode').get_default())
+
+        mf1 = PubForm({'form-prefix-mode': mode})
+        self.assertEqual(mf1.errors, {})
+        m1 = mf1.save(commit=False)
+        self.assertEqual(m1.mode, mode)
+
 
 class FieldOverridesByFormMetaForm(forms.ModelForm):
     class Meta: