[3.1.x] Fixed #31742 -- Fixed makemigrations crash on ForeignKey to an app with mixed case label.

Regression in 9e1b6b8a66af4c2197e5b1b41eb9dbb36e4f6502.

Thanks Ignacio Santolin for the report.
Backport of 62d85a283500e9abb0e1c9ec53c59be468f056a0 from master

django/db/models/fields/related.py

@@ -582,7 +582,11 @@ class ForeignObject(RelatedField):
         if self.remote_field.parent_link:
             kwargs['parent_link'] = self.remote_field.parent_link
         if isinstance(self.remote_field.model, str):
-            kwargs['to'] = self.remote_field.model.lower()
+            if '.' in self.remote_field.model:
+                app_label, model_name = self.remote_field.model.split('.')
+                kwargs['to'] = '%s.%s' % (app_label, model_name.lower())
+            else:
+                kwargs['to'] = self.remote_field.model.lower()
         else:
             kwargs['to'] = self.remote_field.model._meta.label_lower
         # If swappable is True, then see if we're actually pointing to the target

tests/migrations/test_state.py

@@ -867,6 +867,34 @@ class StateTests(SimpleTestCase):
         with self.assertRaisesMessage(ValueError, msg):
             project_state.apps
 
+    def test_reference_mixed_case_app_label(self):
+        new_apps = Apps()
+
+        class Author(models.Model):
+            class Meta:
+                app_label = 'MiXedCase_migrations'
+                apps = new_apps
+
+        class Book(models.Model):
+            author = models.ForeignKey(Author, models.CASCADE)
+
+            class Meta:
+                app_label = 'MiXedCase_migrations'
+                apps = new_apps
+
+        class Magazine(models.Model):
+            authors = models.ManyToManyField(Author)
+
+            class Meta:
+                app_label = 'MiXedCase_migrations'
+                apps = new_apps
+
+        project_state = ProjectState()
+        project_state.add_model(ModelState.from_model(Author))
+        project_state.add_model(ModelState.from_model(Book))
+        project_state.add_model(ModelState.from_model(Magazine))
+        self.assertEqual(len(project_state.apps.get_models()), 3)
+
     def test_real_apps(self):
         """
         Including real apps can resolve dangling FK errors.