Merge pull request piccolo-orm#40 from piccolo-orm/issue_39_empty_date_column_migration_fix

Issue 39 empty date column migration fix

Author: dantownsend

piccolo/columns/__init__.py

@@ -1,5 +1,6 @@
 from .column_types import (  # noqa: F401
     Boolean,
+    Date,
     Decimal,
     Float,
     ForeignKey,

piccolo/query/methods/alter.py

@@ -88,7 +88,7 @@ def querystring(self) -> QueryString:
 class DropDefault(AlterColumnStatement):
     @property
     def querystring(self) -> QueryString:
-        return QueryString("DROP DEFAULT {}", self.column_name)
+        return QueryString(f"ALTER COLUMN {self.column_name} DROP DEFAULT")
 
 
 @dataclass

tests/apps/migrations/auto/test_migration_manager.py

@@ -336,6 +336,65 @@ def test_alter_column_set_default(self):
             [{"column_default": "''::character varying"}],
         )
 
+    @postgres_only
+    def test_alter_column_drop_default(self):
+        """
+        Test setting a column default to None with MigrationManager.
+        """
+        # Make sure it has a non-null default to start with.
+        manager_1 = MigrationManager()
+        manager_1.alter_column(
+            table_class_name="Manager",
+            tablename="manager",
+            column_name="name",
+            params={"default": "Mr Manager"},
+            old_params={"default": None},
+        )
+        asyncio.run(manager_1.run())
+        self.assertEqual(
+            self._get_column_default(),
+            [{"column_default": "'Mr Manager'::character varying"}],
+        )
+
+        # Drop the default.
+        manager_2 = MigrationManager()
+        manager_2.alter_column(
+            table_class_name="Manager",
+            tablename="manager",
+            column_name="name",
+            params={"default": None},
+            old_params={"default": "Mr Manager"},
+        )
+        asyncio.run(manager_2.run())
+        self.assertEqual(
+            self._get_column_default(), [{"column_default": None}],
+        )
+
+        # And add it back once more to be sure.
+        manager_3 = manager_1
+        asyncio.run(manager_3.run())
+        self.assertEqual(
+            self._get_column_default(),
+            [{"column_default": "'Mr Manager'::character varying"}],
+        )
+
+        # Run them all backwards
+        asyncio.run(manager_3.run_backwards())
+        self.assertEqual(
+            self._get_column_default(), [{"column_default": None}],
+        )
+
+        asyncio.run(manager_2.run_backwards())
+        self.assertEqual(
+            self._get_column_default(),
+            [{"column_default": "'Mr Manager'::character varying"}],
+        )
+
+        asyncio.run(manager_1.run_backwards())
+        self.assertEqual(
+            self._get_column_default(), [{"column_default": None}],
+        )
+
     @postgres_only
     def test_alter_column_add_index(self):
         """