Fixed #13776 -- Fixed ModelForm.is_valid() exception with non-nullable FK and blank=True.

Thanks peterbe for the report.

django/forms/models.py

@@ -401,10 +401,12 @@ class BaseModelForm(BaseForm):
 
     def _post_clean(self):
         opts = self._meta
-        # Update the model instance with self.cleaned_data.
-        self.instance = construct_instance(self, self.instance, opts.fields, opts.exclude)
 
         exclude = self._get_validation_exclusions()
+        # a subset of `exclude` which won't have the InlineForeignKeyField
+        # if we're adding a new object since that value doesn't exist
+        # until after the new instance is saved to the database.
+        construct_instance_exclude = list(exclude)
 
         # Foreign Keys being used to represent inline relationships
         # are excluded from basic field value validation. This is for two
@@ -415,8 +417,13 @@ class BaseModelForm(BaseForm):
         # so this can't be part of _get_validation_exclusions().
         for name, field in self.fields.items():
             if isinstance(field, InlineForeignKeyField):
+                if self.cleaned_data.get(name) is not None and self.cleaned_data[name]._state.adding:
+                    construct_instance_exclude.append(name)
                 exclude.append(name)
 
+        # Update the model instance with self.cleaned_data.
+        self.instance = construct_instance(self, self.instance, opts.fields, construct_instance_exclude)
+
         try:
             self.instance.full_clean(exclude=exclude, validate_unique=False)
         except ValidationError as e:

tests/model_forms/models.py

@@ -401,3 +401,9 @@ class Character(models.Model):
 class StumpJoke(models.Model):
     most_recently_fooled = models.ForeignKey(Character, limit_choices_to=today_callable_dict, related_name="+")
     has_fooled_today = models.ManyToManyField(Character, limit_choices_to=today_callable_q, related_name="+")
+
+
+# Model for #13776
+class Student(models.Model):
+    character = models.ForeignKey(Character)
+    study = models.CharField(max_length=30)

tests/model_forms/tests.py

@@ -23,7 +23,7 @@ from .models import (Article, ArticleStatus, Author, Author1, BetterWriter, BigI
     ImprovedArticle, ImprovedArticleWithParentLink, Inventory, Person, Post, Price,
     Product, Publication, TextFile, Triple, Writer, WriterProfile,
     Colour, ColourfulItem, DateTimePost, CustomErrorMessage,
-    test_images, StumpJoke, Character)
+    test_images, StumpJoke, Character, Student)
 
 if test_images:
     from .models import ImageFile, OptionalImageFile
@@ -199,6 +199,34 @@ class ModelFormBaseTest(TestCase):
         instance = construct_instance(form, Person(), fields=())
         self.assertEqual(instance.name, '')
 
+    def test_blank_with_null_foreign_key_field(self):
+        """
+        #13776 -- ModelForm's with models having a FK set to null=False and
+        required=False should be valid.
+        """
+        class FormForTestingIsValid(forms.ModelForm):
+            class Meta:
+                model = Student
+                fields = '__all__'
+
+            def __init__(self, *args, **kwargs):
+                super(FormForTestingIsValid, self).__init__(*args, **kwargs)
+                self.fields['character'].required = False
+
+        char = Character.objects.create(username='user',
+                                        last_action=datetime.datetime.today())
+        data = {'study': 'Engineering'}
+        data2 = {'study': 'Engineering', 'character': char.pk}
+
+        # form is valid because required=False for field 'character'
+        f1 = FormForTestingIsValid(data)
+        self.assertTrue(f1.is_valid())
+
+        f2 = FormForTestingIsValid(data2)
+        self.assertTrue(f2.is_valid())
+        obj = f2.save()
+        self.assertEqual(obj.character, char)
+
     def test_missing_fields_attribute(self):
         message = (
             "Creating a ModelForm without either the 'fields' attribute "