Fixed #31826 -- Made AlterField operation a noop when adding db_column.

AlterField operation with adding a db_column is a noop if the column
name is not changed.

django/db/backends/base/schema.py

@@ -1037,10 +1037,16 @@ class BaseDatabaseSchemaEditor:
         return output
 
     def _field_should_be_altered(self, old_field, new_field):
-        # Don't alter when changing only a field name.
+        _, old_path, old_args, old_kwargs = old_field.deconstruct()
+        _, new_path, new_args, new_kwargs = new_field.deconstruct()
+        # Don't alter when:
+        # - changing only a field name
+        # - adding only a db_column and the column name is not changed
+        old_kwargs.pop('db_column', None)
+        new_kwargs.pop('db_column', None)
         return (
             old_field.column != new_field.column or
-            old_field.deconstruct()[1:] != new_field.deconstruct()[1:]
+            (old_path, old_args, old_kwargs) != (new_path, new_args, new_kwargs)
         )
 
     def _field_should_be_indexed(self, model, field):

tests/migrations/test_operations.py

@@ -1354,6 +1354,59 @@ class OperationTests(OperationTestBase):
         self.assertEqual(definition[1], [])
         self.assertEqual(sorted(definition[2]), ["field", "model_name", "name"])
 
+    def test_alter_field_add_db_column_noop(self):
+        """
+        AlterField operation is a noop when adding only a db_column and the
+        column name is not changed.
+        """
+        app_label = 'test_afadbn'
+        project_state = self.set_up_test_model(app_label, related_model=True)
+        pony_table = '%s_pony' % app_label
+        new_state = project_state.clone()
+        operation = migrations.AlterField('Pony', 'weight', models.FloatField(db_column='weight'))
+        operation.state_forwards(app_label, new_state)
+        self.assertIsNone(
+            project_state.models[app_label, 'pony'].fields['weight'].db_column,
+        )
+        self.assertEqual(
+            new_state.models[app_label, 'pony'].fields['weight'].db_column,
+            'weight',
+        )
+        self.assertColumnExists(pony_table, 'weight')
+        with connection.schema_editor() as editor:
+            with self.assertNumQueries(0):
+                operation.database_forwards(app_label, editor, project_state, new_state)
+        self.assertColumnExists(pony_table, 'weight')
+        with connection.schema_editor() as editor:
+            with self.assertNumQueries(0):
+                operation.database_backwards(app_label, editor, new_state, project_state)
+        self.assertColumnExists(pony_table, 'weight')
+
+        rider_table = '%s_rider' % app_label
+        new_state = project_state.clone()
+        operation = migrations.AlterField(
+            'Rider',
+            'pony',
+            models.ForeignKey('Pony', models.CASCADE, db_column='pony_id'),
+        )
+        operation.state_forwards(app_label, new_state)
+        self.assertIsNone(
+            project_state.models[app_label, 'rider'].fields['pony'].db_column,
+        )
+        self.assertIs(
+            new_state.models[app_label, 'rider'].fields['pony'].db_column,
+            'pony_id',
+        )
+        self.assertColumnExists(rider_table, 'pony_id')
+        with connection.schema_editor() as editor:
+            with self.assertNumQueries(0):
+                operation.database_forwards(app_label, editor, project_state, new_state)
+        self.assertColumnExists(rider_table, 'pony_id')
+        with connection.schema_editor() as editor:
+            with self.assertNumQueries(0):
+                operation.database_forwards(app_label, editor, new_state, project_state)
+        self.assertColumnExists(rider_table, 'pony_id')
+
     def test_alter_field_pk(self):
         """
         Tests the AlterField operation on primary keys (for things like PostgreSQL's SERIAL weirdness)