Fixed #20522 - Allowed use of partially validated object in ModelAdmin.add_view formset validation.

Updated ModelAdmin to use form.instance when passing parent model to
child inlines for add_view. There is effectively no change in the
change_view since the previously passed 'obj' is the same as form.instance.

Thanks to meshy for report, and EvilDMP and timo for review.

tests/admin_views/models.py

@@ -712,6 +712,26 @@ class Choice(models.Model):
         choices=((1, 'Yes'), (0, 'No'), (None, 'No opinion')))
 
 
+class ParentWithDependentChildren(models.Model):
+    """
+    Issue #20522
+    Model where the validation of child foreign-key relationships depends
+    on validation of the parent
+    """
+    some_required_info = models.PositiveIntegerField()
+    family_name = models.CharField(max_length=255, blank=False)
+
+
+class DependentChild(models.Model):
+    """
+    Issue #20522
+    Model that depends on validation of the parent class for one of its
+    fields to validate during clean
+    """
+    parent = models.ForeignKey(ParentWithDependentChildren)
+    family_name = models.CharField(max_length=255)
+
+
 class _Manager(models.Manager):
     def get_queryset(self):
         return super(_Manager, self).get_queryset().filter(pk__gt=1)