Add AlterField and RenameField operations

django/db/migrations/operations/__init__.py

@@ -1,2 +1,2 @@
 from .models import CreateModel, DeleteModel, AlterModelTable
-from .fields import AddField, RemoveField
+from .fields import AddField, RemoveField, AlterField, RenameField

django/db/migrations/operations/fields.py

@@ -54,3 +54,71 @@ def database_backwards(self, app_label, schema_editor, from_state, to_state):
 
     def describe(self):
         return "Remove field %s from %s" % (self.name, self.model_name)
+
+
+class AlterField(Operation):
+    """
+    Alters a field's database column (e.g. null, max_length) to the provided new field
+    """
+
+    def __init__(self, model_name, name, field):
+        self.model_name = model_name
+        self.name = name
+        self.field = field
+
+    def state_forwards(self, app_label, state):
+        state.models[app_label, self.model_name.lower()].fields = [
+            (n, self.field if n == self.name else f) for n, f in state.models[app_label, self.model_name.lower()].fields
+        ]
+
+    def database_forwards(self, app_label, schema_editor, from_state, to_state):
+        from_model = from_state.render().get_model(app_label, self.model_name)
+        to_model = to_state.render().get_model(app_label, self.model_name)
+        schema_editor.alter_field(
+            from_model,
+            from_model._meta.get_field_by_name(self.name)[0],
+            to_model._meta.get_field_by_name(self.name)[0],
+        )
+
+    def database_backwards(self, app_label, schema_editor, from_state, to_state):
+        self.database_forwards(app_label, schema_editor, from_state, to_state)
+
+    def describe(self):
+        return "Alter field %s on %s" % (self.name, self.model_name)
+
+
+class RenameField(Operation):
+    """
+    Renames a field on the model. Might affect db_column too.
+    """
+
+    def __init__(self, model_name, old_name, new_name):
+        self.model_name = model_name
+        self.old_name = old_name
+        self.new_name = new_name
+
+    def state_forwards(self, app_label, state):
+        state.models[app_label, self.model_name.lower()].fields = [
+            (self.new_name if n == self.old_name else n, f) for n, f in state.models[app_label, self.model_name.lower()].fields
+        ]
+
+    def database_forwards(self, app_label, schema_editor, from_state, to_state):
+        from_model = from_state.render().get_model(app_label, self.model_name)
+        to_model = to_state.render().get_model(app_label, self.model_name)
+        schema_editor.alter_field(
+            from_model,
+            from_model._meta.get_field_by_name(self.old_name)[0],
+            to_model._meta.get_field_by_name(self.new_name)[0],
+        )
+
+    def database_backwards(self, app_label, schema_editor, from_state, to_state):
+        from_model = from_state.render().get_model(app_label, self.model_name)
+        to_model = to_state.render().get_model(app_label, self.model_name)
+        schema_editor.alter_field(
+            from_model,
+            from_model._meta.get_field_by_name(self.new_name)[0],
+            to_model._meta.get_field_by_name(self.old_name)[0],
+        )
+
+    def describe(self):
+        return "Rename field %s on %s to %s" % (self.old_name, self.model_name, self.new_name)

django/db/migrations/state.py

@@ -93,10 +93,17 @@ def from_model(cls, model):
 
     def clone(self):
         "Returns an exact copy of this ModelState"
+        # We deep-clone the fields using deconstruction
+        fields = []
+        for name, field in self.fields:
+            _, path, args, kwargs = field.deconstruct()
+            field_class = import_by_path(path)
+            fields.append((name, field_class(*args, **kwargs)))
+        # Now make a copy
         return self.__class__(
             app_label = self.app_label,
             name = self.name,
-            fields = list(self.fields),
+            fields = fields,
             options = dict(self.options),
             bases = self.bases,
         )

tests/migrations/test_operations.py

@@ -22,6 +22,12 @@ def assertColumnExists(self, table, column):
     def assertColumnNotExists(self, table, column):
         self.assertNotIn(column, [c.name for c in connection.introspection.get_table_description(connection.cursor(), table)])
 
+    def assertColumnNull(self, table, column):
+        self.assertEqual([c.null_ok for c in connection.introspection.get_table_description(connection.cursor(), table) if c.name == column][0], True)
+
+    def assertColumnNotNull(self, table, column):
+        self.assertEqual([c.null_ok for c in connection.introspection.get_table_description(connection.cursor(), table) if c.name == column][0], False)
+
     def set_up_test_model(self, app_label):
         """
         Creates a test model state and database table.
@@ -50,7 +56,7 @@ def test_create_model(self):
             "Pony",
             [
                 ("id", models.AutoField(primary_key=True)),
-                ("pink", models.BooleanField(default=True)),
+                ("pink", models.IntegerField(default=1)),
             ],
         )
         # Test the state alteration
@@ -157,3 +163,48 @@ def test_alter_model_table(self):
             operation.database_backwards("test_almota", editor, new_state, project_state)
         self.assertTableExists("test_almota_pony")
         self.assertTableNotExists("test_almota_pony_2")
+
+    def test_alter_field(self):
+        """
+        Tests the AlterField operation.
+        """
+        project_state = self.set_up_test_model("test_alfl")
+        # Test the state alteration
+        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)
+        # Test the database alteration
+        self.assertColumnNotNull("test_alfl_pony", "pink")
+        with connection.schema_editor() as editor:
+            operation.database_forwards("test_alfl", editor, project_state, new_state)
+        self.assertColumnNull("test_alfl_pony", "pink")
+        # And test reversal
+        with connection.schema_editor() as editor:
+            operation.database_backwards("test_alfl", editor, new_state, project_state)
+        self.assertColumnNotNull("test_alfl_pony", "pink")
+
+    def test_rename_field(self):
+        """
+        Tests the RenameField operation.
+        """
+        project_state = self.set_up_test_model("test_rnfl")
+        # Test the state alteration
+        operation = migrations.RenameField("Pony", "pink", "blue")
+        new_state = project_state.clone()
+        operation.state_forwards("test_rnfl", new_state)
+        self.assertIn("blue", [n for n, f in new_state.models["test_rnfl", "pony"].fields])
+        self.assertNotIn("pink", [n for n, f in new_state.models["test_rnfl", "pony"].fields])
+        # Test the database alteration
+        self.assertColumnExists("test_rnfl_pony", "pink")
+        self.assertColumnNotExists("test_rnfl_pony", "blue")
+        with connection.schema_editor() as editor:
+            operation.database_forwards("test_rnfl", editor, project_state, new_state)
+        self.assertColumnExists("test_rnfl_pony", "blue")
+        self.assertColumnNotExists("test_rnfl_pony", "pink")
+        # And test reversal
+        with connection.schema_editor() as editor:
+            operation.database_backwards("test_rnfl", editor, new_state, project_state)
+        self.assertColumnExists("test_rnfl_pony", "pink")
+        self.assertColumnNotExists("test_rnfl_pony", "blue")