Autodetect field alters

django/db/migrations/autodetector.py

@@ -89,6 +89,20 @@ class MigrationAutodetector(object):
                         name = field_name,
                     )
                 )
+            # The same fields
+            for field_name in old_field_names.intersection(new_field_names):
+                # Did the field change?
+                old_field_dec = old_model_state.get_field_by_name(field_name).deconstruct()
+                new_field_dec = new_model_state.get_field_by_name(field_name).deconstruct()
+                if old_field_dec != new_field_dec:
+                    self.add_to_migration(
+                        app_label,
+                        operations.AlterField(
+                            model_name = model_name,
+                            name = field_name,
+                            field = new_model_state.get_field_by_name(field_name),
+                        )
+                    )
         # Alright, now add internal dependencies
         for app_label, migrations in self.migrations.items():
             for m1, m2 in zip(migrations, migrations[1:]):

django/db/migrations/state.py

@@ -126,3 +126,9 @@ class ModelState(object):
             tuple(self.bases),
             body,
         )
+
+    def get_field_by_name(self, name):
+        for fname, field in self.fields:
+            if fname == name:
+                return field
+        raise ValueError("No field called %s on model %s" % (name, self.name))

tests/migrations/test_autodetector.py

@@ -13,6 +13,7 @@ class AutodetectorTests(TestCase):
 
     author_empty = ModelState("testapp", "Author", [("id", models.AutoField(primary_key=True))])
     author_name = ModelState("testapp", "Author", [("id", models.AutoField(primary_key=True)), ("name", models.CharField(max_length=200))])
+    author_name_longer = ModelState("testapp", "Author", [("id", models.AutoField(primary_key=True)), ("name", models.CharField(max_length=400))])
     other_pony = ModelState("otherapp", "Pony", [("id", models.AutoField(primary_key=True))])
     other_stable = ModelState("otherapp", "Stable", [("id", models.AutoField(primary_key=True))])
     third_thing = ModelState("thirdapp", "Thing", [("id", models.AutoField(primary_key=True))])
@@ -130,3 +131,20 @@ class AutodetectorTests(TestCase):
         action = migration.operations[0]
         self.assertEqual(action.__class__.__name__, "RemoveField")
         self.assertEqual(action.name, "name")
+
+    def test_alter_field(self):
+        "Tests autodetection of new fields"
+        # Make state
+        before = self.make_project_state([self.author_name])
+        after = self.make_project_state([self.author_name_longer])
+        autodetector = MigrationAutodetector(before, after)
+        changes = autodetector.changes()
+        # Right number of migrations?
+        self.assertEqual(len(changes['testapp']), 1)
+        # Right number of actions?
+        migration = changes['testapp'][0]
+        self.assertEqual(len(migration.operations), 1)
+        # Right action?
+        action = migration.operations[0]
+        self.assertEqual(action.__class__.__name__, "AlterField")
+        self.assertEqual(action.name, "name")

tests/migrations/test_operations.py

@@ -173,8 +173,8 @@ class OperationTests(TestCase):
         operation = migrations.AlterField("Pony", "pink", models.IntegerField(null=True))
         new_state = project_state.clone()
         operation.state_forwards("test_alfl", new_state)
-        self.assertEqual([f for n, f in project_state.models["test_alfl", "pony"].fields if n == "pink"][0].null, False)
-        self.assertEqual([f for n, f in new_state.models["test_alfl", "pony"].fields if n == "pink"][0].null, True)
+        self.assertEqual(project_state.models["test_alfl", "pony"].get_field_by_name("pink").null, False)
+        self.assertEqual(new_state.models["test_alfl", "pony"].get_field_by_name("pink").null, True)
         # Test the database alteration
         self.assertColumnNotNull("test_alfl_pony", "pink")
         with connection.schema_editor() as editor: