def upgrade():
    """Upgrade database."""
    # table ObjectVersion: modify primary_key
    if op.get_context().dialect.name == 'mysql':
        Fk = 'fk_files_object_bucket_id_files_bucket'
        op.execute(
            'ALTER TABLE files_object '
            'DROP FOREIGN KEY {0}, DROP PRIMARY KEY, '
            'ADD PRIMARY KEY(version_id), '
            'ADD FOREIGN KEY(bucket_id) '
            'REFERENCES files_bucket(id)'.format(Fk))
    else:
        op.drop_constraint('pk_files_object', 'files_object', type_='primary')
        op.create_primary_key(
            'pk_files_object', 'files_object', ['version_id'])
    op.create_unique_constraint(
        'uq_files_object_bucket_id', 'files_object',
        ['bucket_id', 'version_id', 'key'])
    # table ObjectVersionTag: create
    op.create_table(
        'files_objecttags',
        sa.Column(
            'version_id',
            sqlalchemy_utils.types.uuid.UUIDType(),
            nullable=False),
        sa.Column(
            'key',
            sa.Text().with_variant(mysql.VARCHAR(255), 'mysql'),
            nullable=False
        ),
        sa.Column(
            'value',
            sa.Text().with_variant(mysql.VARCHAR(255), 'mysql'),
            nullable=False
        ),
        sa.PrimaryKeyConstraint('version_id', 'key'),
        sa.ForeignKeyConstraint(
            ['version_id'],
            [u'files_object.version_id'],
            ondelete='CASCADE'),
    )
Upgrade database.