[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

django/db/migrations/state.py

@@ -143,6 +143,12 @@ class ModelState(object):
         # Sanity-check that fields is NOT a dict. It must be ordered.
         if isinstance(self.fields, dict):
             raise ValueError("ModelState.fields cannot be a dict - it must be a list of 2-tuples.")
+        # Sanity-check that fields are NOT already bound to a model.
+        for name, field in fields:
+            if hasattr(field, 'model'):
+                raise ValueError(
+                    'ModelState.fields cannot be bound to a model - "%s" is.' % name
+                )
 
     @classmethod
     def from_model(cls, model):
@@ -226,19 +232,19 @@ class ModelState(object):
             bases,
         )
 
-    def clone(self):
-        "Returns an exact copy of this ModelState"
-        # We deep-clone the fields using deconstruction
-        fields = []
+    def construct_fields(self):
+        "Deep-clone the fields using deconstruction"
         for name, field in self.fields:
             _, path, args, kwargs = field.deconstruct()
             field_class = import_string(path)
-            fields.append((name, field_class(*args, **kwargs)))
-        # Now make a copy
+            yield name, field_class(*args, **kwargs)
+
+    def clone(self):
+        "Returns an exact copy of this ModelState"
         return self.__class__(
             app_label=self.app_label,
             name=self.name,
-            fields=fields,
+            fields=list(self.construct_fields()),
             options=dict(self.options),
             bases=self.bases,
         )
@@ -260,7 +266,7 @@ class ModelState(object):
         except LookupError:
             raise InvalidBasesError("Cannot resolve one or more bases from %r" % (self.bases,))
         # Turn fields into a dict for the body, add other bits
-        body = dict(self.fields)
+        body = dict(self.construct_fields())
         body['Meta'] = meta
         body['__module__'] = "__fake__"
         # Then, make a Model object

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"

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)