Fixed #25349 -- Allowed a ModelForm to unset a fields with blank=True, required=False.

django/forms/models.py

@@ -376,11 +376,6 @@ class BaseModelForm(BaseForm):
 
         exclude = self._get_validation_exclusions()
 
-        try:
-            self.instance = construct_instance(self, self.instance, opts.fields, exclude)
-        except ValidationError as e:
-            self._update_errors(e)
-
         # Foreign Keys being used to represent inline relationships
         # are excluded from basic field value validation. This is for two
         # reasons: firstly, the value may not be supplied (#12507; the
@@ -392,6 +387,11 @@ class BaseModelForm(BaseForm):
             if isinstance(field, InlineForeignKeyField):
                 exclude.append(name)
 
+        try:
+            self.instance = construct_instance(self, self.instance, opts.fields, opts.exclude)
+        except ValidationError as e:
+            self._update_errors(e)
+
         try:
             self.instance.full_clean(exclude=exclude, validate_unique=False)
         except ValidationError as e:

tests/model_forms/models.py

@@ -474,3 +474,9 @@ class StrictAssignmentAll(models.Model):
         if self._should_error is True:
             raise ValidationError(message="Cannot set attribute", code='invalid')
         super(StrictAssignmentAll, self).__setattr__(key, value)
+
+
+# A model with ForeignKey(blank=False, null=True)
+class Award(models.Model):
+    name = models.CharField(max_length=30)
+    character = models.ForeignKey(Character, models.SET_NULL, blank=False, null=True)

tests/model_forms/tests.py

@@ -23,7 +23,7 @@ from django.utils import six
 from django.utils._os import upath
 
 from .models import (
-    Article, ArticleStatus, Author, Author1, BetterWriter, BigInt, Book,
+    Article, ArticleStatus, Author, Author1, Award, BetterWriter, BigInt, Book,
     Category, Character, Colour, ColourfulItem, CommaSeparatedInteger,
     CustomErrorMessage, CustomFF, CustomFieldForExclusionModel, DateTimePost,
     DerivedBook, DerivedPost, Document, ExplicitPK, FilePathModel,
@@ -236,6 +236,40 @@ class ModelFormBaseTest(TestCase):
         obj = f2.save()
         self.assertEqual(obj.character, char)
 
+    def test_blank_false_with_null_true_foreign_key_field(self):
+        """
+        A ModelForm with a model having ForeignKey(blank=False, null=True)
+        and the form field set to required=False should allow the field to be
+        unset.
+        """
+        class AwardForm(forms.ModelForm):
+            class Meta:
+                model = Award
+                fields = '__all__'
+
+            def __init__(self, *args, **kwargs):
+                super(AwardForm, self).__init__(*args, **kwargs)
+                self.fields['character'].required = False
+
+        character = Character.objects.create(username='user', last_action=datetime.datetime.today())
+        award = Award.objects.create(name='Best sprinter', character=character)
+        data = {'name': 'Best tester', 'character': ''}  # remove character
+        form = AwardForm(data=data, instance=award)
+        self.assertTrue(form.is_valid())
+        award = form.save()
+        self.assertIsNone(award.character)
+
+    def test_save_blank_false_with_required_false(self):
+        """
+        A ModelForm with a model with a field set to blank=False and the form
+        field set to required=False should allow the field to be unset.
+        """
+        obj = Writer.objects.create(name='test')
+        form = CustomWriterForm(data={'name': ''}, instance=obj)
+        self.assertTrue(form.is_valid())
+        obj = form.save()
+        self.assertEqual(obj.name, '')
+
     def test_missing_fields_attribute(self):
         message = (
             "Creating a ModelForm without either the 'fields' attribute "