Write records to the data source indicating that a transfer has been started for a particular
    relation.

def start_transfer(self, relation_name):
    """Write records to the data source indicating that a transfer has been started for a particular
    relation.
    """
    self.reset(relation_name)
    relation = Relation(name=relation_name)
    self.data_access.insert_model(relation)
    return relation