Question

我是sqlalchemy的新手。我已经按照教程创建了一个与mysql db

关系的现有数据库的自动化

from sqlalchemy import create_engine, MetaData, Column, Table, ForeignKey
from sqlalchemy.ext.automap import automap_base, generate_relationship
from sqlalchemy.orm import relationship, backref
from config import constr, mytables

def _gen_relationship(base, direction, return_fn,
                  attrname, local_cls, refferred_cls, **kw):
    return generate_relationship(base, direction, return_fn, attrname, local_cls, refferred_cls, **kw)

engine = create_engine(constr)
metadata = MetaData()
metadata.reflect(engine, only=mytables)
Base = automap_base(metadata=metadata)
Base.prepare(engine, reflect=True, generate_relationship=_gen_relationship)
Tableclass1 = Base.classes.table1
Tableclass2 = Base.classes.table2

Table2.ID映射到table1个列之一。但是，当我尝试使用查询并加入table1和table2时，它会报告错误，说“＆＃34;无法找到任何外键关系＆＃34;”。既然我知道了这两个表的关系，那么在创建类实例之后是否可以声明这种关系？或者有没有办法在查询功能中明确告诉这种关系？谢谢！

Answer 1

虽然可以在@mpolednik提到的查询中执行此操作，但如果我正确地阅读了您的问题，理想的解决方案是在您的类中声明关系以供重复使用。

可以通过预先声明您想要使用的类来实现，如下所示：

from sqlalchemy.ext.automap import automap_base
from sqlalchemy.orm import Session, relationship
from sqlalchemy import create_engine, Column, String

Base = automap_base()

engine = create_engine("postgresql://user:pass@localhost:5432/mydb")

# pre-declare User for the 'user' table
class User(Base):
    __tablename__ = 'user'

    # override schema elements like Columns
    oname = Column('originalname', String)

    # and a relationship. I name it 'weird' because in my database schema
    # this relationship makes absolutely no sense, but it does demonstrate
    # the point
    weird = relationship("usergroup",
                         foreign_keys='usergroup.id',
                         primaryjoin='and_(usergroup.id==User.id)')

Base.prepare(engine, reflect=True)
session = Session(engine)

# Test this by querying the User table and then following the relationship
u = session.query(User).filter(User.oname == 'testuser').one()
print (u.oname)
for g in u1.weird:
    print g.name

请参阅此处了解文档（包括其他示例）：http://docs.sqlalchemy.org/en/latest/orm/extensions/automap.html?highlight=automap#specifying-classes-explicitly

是否可以在SqlAlchemy中的automap创建类之后声明关系

1 个答案: