Return the current migration in the database.

def database_current_migration(self):
        """
        Return the current migration in the database.
        """
        # If the table doesn't even exist, return None.
        if not self.migration_table.exists(self.session.bind):
            return None

        # Also return None if self.migration_data is None.
        if self.migration_data is None:
            return None

        return self.migration_data.version