使用默认值初始化sqlalchemy连接对象

Question

如果我有一个sqlalchemy对象，我试图用默认值实例化。它映射到两个表的联接，BusinessObj（）有没有办法在联接不是属性或显式初始化表中字段的情况下调用TableA和TableB的初始化？

在以下示例中，field_1应该自动填充default_field，但根本不填充。

    from pprint import pprint
    from random import randrange
    from sqlalchemy import MetaData, Column, Integer,String, Boolean, ForeignKey, join
    from sqlalchemy.ext.declarative import declarative_base
    from sqlalchemy.orm import column_property, relationship, synonym

    metadata = MetaData()
    Base = declarative_base()

    def new_id():
        return randrange(1000,9999)

    class TableA(Base):
        __tablename__ = 'table_a'

        table_a_id = Column(Integer, primary_key=True)
        field_1 = Column(Integer)

        id = synonym(table_a_id)

        def __init__(self, table_a_id=new_id(), field_1='default_field'):
            self.table_a_id = table_a_id
            self.field_1 = field_1

    class TableB(Base):
        __tablename__ = 'table_b'

        table_b_id = Column(Integer, primary_key=True)
        name = Column(String)

        id = synonym(table_b_id)

        def __init__(self, table_b_id=new_id(), name='default_name'):
            self.table_b_id = table_b_id
            self.name = name

    combo = join(TableA, TableB, onclause=TableA.table_a_id == TableB.table_b_id)

    class BusinessObj(Base):
        __table__ = combo

        id = column_property(TableA.table_a_id, TableB.table_b_id)

        def __init__(self, name='frank', id=new_id()):
            self.name = name
            self.id = id



    new_bus_obj = BusinessObj()

    pprint(vars(new_bus_obj))

    {'_sa_instance_state': <sqlalchemy.orm.state.InstanceState object at 0x03597750>,
     'id': 7599,
     'name': 'frank'}