Allow specification of a server column default in migrations.

Replaces #2803, thanks @b40yd

CHANGELOG.md

@@ -8,6 +8,8 @@ https://github.com/coleifer/peewee/releases
 ## master
 
 * Add bitwise and other helper methods to `BigBitField`, #2802.
+* Add `add_column_default` and `drop_column_default` migrator methods for
+  specifying a server-side default value, #2803.
 * The new `star` attribute was causing issues for users who had a field named
   star on their models. This attribute is now renamed to `__star__`. #2796.
 

docs/peewee/playhouse.rst

@@ -3116,6 +3116,31 @@ Adding or dropping table constraints:
     # Add a UNIQUE constraint on the first and last names.
     migrate(migrator.add_unique('person', 'first_name', 'last_name'))
 
+Adding or dropping a database-level default value for a column:
+
+.. code-block:: python
+
+    # Add a default value for a status column.
+    migrate(migrator.add_column_default(
+        'entries',
+        'status',
+        'draft'))
+
+    # Remove the default.
+    migrate(migrator.drop_column_default('entries', 'status'))
+
+    # Use a function for the default value (does not work with Sqlite):
+    migrate(migrator.add_column_default(
+        'entries',
+        'timestamp',
+        fn.now()))
+
+    # Or alternatively (works with Sqlite):
+    migrate(migrator.add_column_default(
+        'entries',
+        'timestamp',
+        'now()'))
+
 .. note::
     Postgres users may need to set the search-path when using a non-standard
     schema. This can be done as follows:

playhouse/migrate.py

@@ -389,6 +389,32 @@ def drop_not_null(self, table, column):
                 ._alter_column(self.make_context(), table, column)
                 .literal(' DROP NOT NULL'))
 
+    @operation
+    def add_column_default(self, table, column, default):
+        if default is None:
+            raise ValueError('`default` must be not None/NULL.')
+        if callable_(default):
+            default = default()
+        # Try to handle SQL functions and string literals, otherwise pass as a
+        # bound value.
+        if isinstance(default, str) and default.endswith((')', "'")):
+            default = SQL(default)
+
+        return (self
+                ._alter_table(self.make_context(), table)
+                .literal(' ALTER COLUMN ')
+                .sql(Entity(column))
+                .literal(' SET DEFAULT ')
+                .sql(default))
+
+    @operation
+    def drop_column_default(self, table, column):
+        return (self
+                ._alter_table(self.make_context(), table)
+                .literal(' ALTER COLUMN ')
+                .sql(Entity(column))
+                .literal(' DROP DEFAULT'))
+
     @operation
     def alter_column_type(self, table, column, field, cast=None):
         # ALTER TABLE <table> ALTER COLUMN <column>
@@ -866,6 +892,27 @@ def _drop_not_null(column_name, column_def):
             return column_def.replace('NOT NULL', '')
         return self._update_column(table, column, _drop_not_null)
 
+    @operation
+    def add_column_default(self, table, column, default):
+        if default is None:
+            raise ValueError('`default` must be not None/NULL.')
+        if callable_(default):
+            default = default()
+        if (isinstance(default, str) and not default.endswith((')', "'"))
+            and not default.isdigit()):
+            default = "'%s'" % default
+        def _add_default(column_name, column_def):
+            # Try to handle SQL functions and string literals, otherwise quote.
+            return column_def + ' DEFAULT %s' % default
+        return self._update_column(table, column, _add_default)
+
+    @operation
+    def drop_column_default(self, table, column):
+        def _drop_default(column_name, column_def):
+            col = re.sub(r'DEFAULT\s+[\w"\'\(\)]+(\s|$)', '', column_def, re.I)
+            return col.strip()
+        return self._update_column(table, column, _drop_default)
+
     @operation
     def alter_column_type(self, table, column, field, cast=None):
         if cast is not None:

tests/migrations.py

@@ -330,6 +330,29 @@ class Meta:
     def test_rename_gh380_sqlite_legacy(self):
         self.test_rename_gh380(legacy=True)
 
+    def test_add_default_drop_default(self):
+        with self.database.transaction():
+            migrate(self.migrator.add_column_default('person', 'first_name',
+                                                     default='x'))
+
+        p = Person.create(last_name='Last')
+        p_db = Person.get(Person.last_name == 'Last')
+        self.assertEqual(p_db.first_name, 'x')
+
+        with self.database.transaction():
+            migrate(self.migrator.drop_column_default('person', 'first_name'))
+
+        if IS_MYSQL:
+            # MySQL, even though the column is NOT NULL, does not seem to be
+            # enforcing the constraint(?).
+            Person.create(last_name='Last2')
+            p_db = Person.get(Person.last_name == 'Last2')
+            self.assertEqual(p_db.first_name, '')
+        else:
+            with self.assertRaises(IntegrityError):
+                with self.database.transaction():
+                    Person.create(last_name='Last2')
+
     def test_add_not_null(self):
         self._create_people()