refs #134: fix test_alter_size with redshift. Redshift can not decrea…

…se the size of a column that have default.

django_redshift_backend/base.py

@@ -849,13 +849,17 @@ def _get_max_length(field):
 
         old_max_length = _get_max_length(old_field)
         new_max_length = _get_max_length(new_field)
+        decrease_size_with_default = old_default is not None and (
+            old_max_length > new_max_length
+        )
 
         # Size is changed
         if (
             type(old_field) is type(new_field)
             and old_max_length is not None
             and new_max_length is not None
             and old_max_length != new_max_length
+            and not decrease_size_with_default
         ):
             # if shrink size as `old_field.max_length > new_field.max_length` and
             # larger data in database, this change will raise exception.
@@ -921,7 +925,11 @@ def _get_max_length(field):
             fragment = actions.pop(0)
 
         # Type or default is changed?
-        elif (old_type != new_type) or needs_database_default:
+        elif (
+            (old_type != new_type)
+            or needs_database_default
+            or decrease_size_with_default
+        ):
             fragment, actions = self._alter_column_with_recreate(
                 model, old_field, new_field
             )

tests/test_migrations.py

@@ -37,7 +37,40 @@ def execute(self, sql, params=()):
         print('\n'.join(collected_sql))
 
     @postgres_fixture()
-    def test_alter_size(self):
+    def test_alter_size_with_nullable(self):
+        new_state = self.set_up_test_model('test')
+        operations = [
+            migrations.AddField(
+                model_name='Pony',
+                name='name',
+                field=models.CharField(max_length=10, verbose_name='name', null=True),
+            ),
+            migrations.AlterField(
+                model_name='Pony',
+                name='name',
+                field=models.CharField(max_length=20, verbose_name='name', null=True),
+            ),
+            migrations.AlterField(
+                model_name='Pony',
+                name='name',
+                field=models.CharField(max_length=10, verbose_name='name', null=True),
+            ),
+        ]
+
+        with self.collect_sql() as sqls:
+            self.apply_operations('test', new_state, operations)
+
+        self.assertEqual([
+            # add column
+            '''ALTER TABLE "test_pony" ADD COLUMN "name" varchar(10) NULL;''',
+            # increase size
+            '''ALTER TABLE "test_pony" ALTER COLUMN "name" TYPE varchar(20);''',
+            # decrease size
+            '''ALTER TABLE "test_pony" ALTER COLUMN "name" TYPE varchar(10);''',
+        ], sqls)
+
+    @postgres_fixture()
+    def test_alter_size_with_default(self):
         new_state = self.set_up_test_model('test')
         operations = [
             migrations.AddField(
@@ -48,22 +81,28 @@ def test_alter_size(self):
             migrations.AlterField(
                 model_name='Pony',
                 name='name',
-                field=models.CharField(max_length=20, verbose_name='name'),
+                field=models.CharField(max_length=20, verbose_name='name', default=''),
             ),
             migrations.AlterField(
                 model_name='Pony',
                 name='name',
-                field=models.CharField(max_length=10, verbose_name='name'),
+                field=models.CharField(max_length=10, verbose_name='name', default=''),
             ),
         ]
 
         with self.collect_sql() as sqls:
             self.apply_operations('test', new_state, operations)
 
         self.assertEqual([
+            # add column
             '''ALTER TABLE "test_pony" ADD COLUMN "name" varchar(10) DEFAULT '' NOT NULL;''',
+            # increase size
             '''ALTER TABLE "test_pony" ALTER COLUMN "name" TYPE varchar(20);''',
-            '''ALTER TABLE "test_pony" ALTER COLUMN "name" TYPE varchar(10);''',
+            # decrease size
+            '''ALTER TABLE "test_pony" ADD COLUMN "name_tmp" varchar(10) DEFAULT '' NOT NULL;''',
+            '''UPDATE test_pony SET "name_tmp" = "name" WHERE "name" IS NOT NULL;''',
+            '''ALTER TABLE test_pony DROP COLUMN "name" CASCADE;''',
+            '''ALTER TABLE test_pony RENAME COLUMN "name_tmp" TO "name";''',
         ], sqls)
 
     @postgres_fixture()