SQLAlchemy - 将模型子类定义为表的子集

Question

SQLAlchemy新手在这里.

我正在尝试定义一个表示表数据子集的模型子类.具体来说,我希望子类映射给定ID的最新行.

例如,假设我有以下模型:

class AddressHistory(Base):
    __table__ = 'address_table'

    date = Column(Date, index=True, nullable=False)
    id = Column(BigInteger, primary_key=True)
    street = Column(String(2000))
    city = Column(String(2000))
    state = Column(String(2000))
    zip = Column(Integer)

我想要做的是定义这个模型的子类,它代表给定id的最新地址记录:

class MostRecentAddress(Address):
    “””
    Represents a row in AddressHistory with the most recent date for a given id.
    ”””

是否有某种子查询我可以传递给mapper_args？或者有没有办法将表定义为select语句？

Answer 1

您正在寻找单表继承。

https://docs.sqlalchemy.org/en/13/orm/inheritance.html#single-table-inheritance

您的代码示例几乎完全准确地说明了如何执行此操作。您只需要添加映射器即可。

class Person(Base):
    __tablename__ = 'people'
    id = Column(Integer, primary_key=True)
    discriminator = Column('type', String(50))
    __mapper_args__ = {'polymorphic_on': discriminator}

class Engineer(Person):
    __mapper_args__ = {'polymorphic_identity': 'engineer'}
    primary_language = Column(String(50))