[1.7.x] Fixed #22659 -- Prevent model states from sharing field instances.

Thanks to Trac alias tbartelmess for the report and the test project.

Backport of 7a38f88922 from master

2 files changed, 20 insertions, 3 deletions

diff --git a/tests/migrations/test_autodetector.py b/tests/migrations/test_autodetector.py
index 54361f00ed..cceb450452 100644
--- a/tests/migrations/test_autodetector.py
+++ b/tests/migrations/test_autodetector.py
@@ -239,7 +239,7 @@ class AutodetectorTests(TestCase):
         action = migration.operations[0]
         self.assertEqual(action.__class__.__name__, "AlterField")
         self.assertEqual(action.name, "author")
-        self.assertEqual(action.field.rel.to.__name__, "Writer")
+        self.assertEqual(action.field.rel.to, "testapp.Writer")
 
     def test_rename_model_with_renamed_rel_field(self):
         """
@@ -276,7 +276,7 @@ class AutodetectorTests(TestCase):
         action = migration.operations[1]
         self.assertEqual(action.__class__.__name__, "AlterField")
         self.assertEqual(action.name, "writer")
-        self.assertEqual(action.field.rel.to.__name__, "Writer")
+        self.assertEqual(action.field.rel.to, "testapp.Writer")
 
     def test_fk_dependency(self):
         "Tests that having a ForeignKey automatically adds a dependency"
diff --git a/tests/migrations/test_state.py b/tests/migrations/test_state.py
index ab65630105..9be04f3dfc 100644
--- a/tests/migrations/test_state.py
+++ b/tests/migrations/test_state.py
@@ -356,7 +356,7 @@ class StateTests(TestCase):
         project_state = ProjectState()
         project_state.add_model_state(ModelState.from_model(TestModel))
         with self.assertRaises(ValueError):
-            rendered_state = project_state.render()
+            project_state.render()
 
         # If we include the real app it should succeed
         project_state = ProjectState(real_apps=["contenttypes"])
@@ -372,3 +372,20 @@ class ModelStateTests(TestCase):
     def test_custom_model_base(self):
         state = ModelState.from_model(ModelWithCustomBase)
         self.assertEqual(state.bases, (models.Model,))
+
+    def test_bound_field_sanity_check(self):
+        field = models.CharField(max_length=1)
+        field.model = models.Model
+        with self.assertRaisesMessage(ValueError,
+                'ModelState.fields cannot be bound to a model - "field" is.'):
+            ModelState('app', 'Model', [('field', field)])
+
+    def test_fields_immutability(self):
+        """
+        Tests that rendering a model state doesn't alter its internal fields.
+        """
+        apps = Apps()
+        field = models.CharField(max_length=1)
+        state = ModelState('app', 'Model', [('name', field)])
+        Model = state.render(apps)
+        self.assertNotEqual(Model._meta.get_field('name'), field)