python - sql炼金术: Could not determine join condition between parent/child tables

Question

我是 SQLAlchemy 的新手。我在我的应用程序中的两个模型之间建立一对多关系时遇到了一些困难。我有两个模型用户“照片”。一个用户只有一个与之相关联的角色，一个角色有多个相关联的用户。

这是我的 data_generator.py 文件中的代码:

# coding=utf-8
from sqlalchemy import Column, Integer, String, BigInteger,Date, Enum, ForeignKey
from sqlalchemy.ext.declarative import declarative_base
import time
from sqlalchemy import create_engine
from sqlalchemy.orm import sessionmaker, relationship
import datetime

Base = declarative_base()


class User(Base):

    __tablename__ = 'users'

    id = Column(Integer(), primary_key=True)
    username = Column(String(30), unique=True, nullable=False)  
    password = Column(String, default='123456', nullable=False) 
    name = Column(String(30), nullable=False) 
    grade = Column(String(30))  
    emp_no = Column(BigInteger, unique=True, nullable=False) 
    roles = relationship('Role', back_populates='users')

class Scene(Base):

    __tablename__ = 'scenes'

    id = Column(Integer, primary_key=True)
    scene_name = Column(String(30), nullable=False) 
    life_time = Column(Date, nullable=False,
                       default=datetime.datetime.strptime(
                       time.strftime("%Y-%m-%d", time.localtime(time.time() + (12 * 30 * 24 * 3600))),'%Y-%m-%d').date())  
    scene_description = Column(String(150), default="")  


class Gateway(Base):

    __tablename__ = 'gateways'

    id = Column(Integer, primary_key=True)
    gateway_name = Column(String(30), nullable=False) 
    gateway_api_key = Column(String(100), nullable=False, unique=True) 
    gateway_type = Column(Enum('up', 'down', 'soft', name="gateway_type"), nullable=False)


class Role(Base):

    __tablename__ = 'roles'

    id = Column(Integer, primary_key=True)

    role_name = Column(String(30), unique=True, nullable=False)


    users = relationship('User', back_populates='roles')

    def __repr__(self):
        return self.role_name







engine = create_engine('sqlite:///memory:')
Session = sessionmaker()
Session.configure(bind=engine)
session = Session()
Base.metadata.create_all(engine)

ed_user = User(name='ed', username='jack', password='123', emp_no=1, grade='1', roles=1)
example_scene = Scene(scene_name='example_1', scene_description='example_description')
example_gateway = Gateway(gateway_name='example_1',gateway_api_key='11111',gateway_type='up')

# session.add(example_gateway)
# session.commit()


def init_user(flag, number):
    while number >= 1:
        if flag == 1:
            ed_user = User(name='ed', username='jack', password='123', emp_no=1, grade='1')
            pass
        if flag == 2:
            # TODO admin
            pass
        if flag == 3:
            # TODO teacher
            pass
        number -= 1


def init_scene(number):
    while number >= 1:
        number -= 1
        # TODO scene

def init_gateway(api_key, number):
    # TODO gateway
    pass

if __name__ == '__main__':
    with session.no_autoflush:
        c = session.query(Gateway).all()
    print c[0].id

我不断遇到的错误如下所示:

/usr/bin/python2.7 /home/pajamas/PycharmProjects/untitled5/data_generator.py
Traceback (most recent call last):
  File "/home/pajamas/PycharmProjects/untitled5/data_generator.py", line 73, in <module>
    ed_user = User(name='ed', username='jack', password='123', emp_no=1, grade='1', roles=1)
  File "<string>", line 2, in __init__
  File "/home/pajamas/.local/lib/python2.7/site-packages/sqlalchemy/orm/instrumentation.py", line 347, in _new_state_if_none
    state = self._state_constructor(instance, self)
  File "/home/pajamas/.local/lib/python2.7/site-packages/sqlalchemy/util/langhelpers.py", line 764, in __get__
    obj.__dict__[self.__name__] = result = self.fget(obj)
  File "/home/pajamas/.local/lib/python2.7/site-packages/sqlalchemy/orm/instrumentation.py", line 177, in _state_constructor
    self.dispatch.first_init(self, self.class_)
  File "/home/pajamas/.local/lib/python2.7/site-packages/sqlalchemy/event/attr.py", line 256, in __call__
    fn(*args, **kw)
  File "/home/pajamas/.local/lib/python2.7/site-packages/sqlalchemy/orm/mapper.py", line 3088, in _event_on_first_init
    configure_mappers()
  File "/home/pajamas/.local/lib/python2.7/site-packages/sqlalchemy/orm/mapper.py", line 2984, in configure_mappers
    mapper._post_configure_properties()
  File "/home/pajamas/.local/lib/python2.7/site-packages/sqlalchemy/orm/mapper.py", line 1810, in _post_configure_properties
    prop.init()
  File "/home/pajamas/.local/lib/python2.7/site-packages/sqlalchemy/orm/interfaces.py", line 184, in init
    self.do_init()
  File "/home/pajamas/.local/lib/python2.7/site-packages/sqlalchemy/orm/relationships.py", line 1658, in do_init
    self._setup_join_conditions()
  File "/home/pajamas/.local/lib/python2.7/site-packages/sqlalchemy/orm/relationships.py", line 1733, in _setup_join_conditions
    can_be_synced_fn=self._columns_are_mapped
  File "/home/pajamas/.local/lib/python2.7/site-packages/sqlalchemy/orm/relationships.py", line 1991, in __init__
    self._determine_joins()
  File "/home/pajamas/.local/lib/python2.7/site-packages/sqlalchemy/orm/relationships.py", line 2096, in _determine_joins
    "specify a 'primaryjoin' expression." % self.prop)
sqlalchemy.exc.NoForeignKeysError: Could not determine join condition between parent/child tables on relationship User.roles - there are no foreign keys linking these tables.  Ensure that referencing columns are associated with a ForeignKey or ForeignKeyConstraint, or specify a 'primaryjoin' expression.

Process finished with exit code 1

有人可以帮助我吗？将不胜感激。

Answer 1

User和Role之间可能存在三种关系:

• 一对一(一个用户只有一个角色)
• 多对一(一个用户有多个角色)
• 多对多(许多用户有很多角色)

对于一对一:

class Role(Base):

    id = Column(Integer, primary_key=True)

    # ...

    user_id = Column(Integer, ForeignKey("user.id"))

class User(Base):

    id = Column(Integer, primary_key=True)

    # ...

    role = relationship("Role", back_populates="user", uselist=False)

对于多对一:

class Role(Base):

    id = Column(Integer, primary_key=True)

    # ...

    user_id = Column(Integer, ForeignKey("user.id"))

class User(Base):

    id = Column(Integer, primary_key=True)

    # ...

    roles = relationship("Role", back_populates="user")

对于多对多:(在这种关系中，我们需要一个关联表)

roles_users = Table("roles_users", 
Column("role_id", Integer, ForeignKey("role.id")), 
Column("user_id", Integer, ForeignKey("user.id")))

class Role(Base):

    id = Column(Integer, primary_key=True)

    # ...

class User(Base):

    id = Column(Integer, primary_key=True)

    # ...

    roles = relationship("Role", back_populates="users", secondary=roles_users)