如何修复烧瓶中的sqlalchemy.exc.NoForeignKeysError？

Question

I have this error:

sqlalchemy.exc.NoForeignKeysError: Could not determine join condition between parent/child tables on relationship User.car - there are no foreign keys linking these tables via secondary table 'user'. Ensure that referencing columns are associated with a ForeignKey or ForeignKeyConstraint, or specify 'primaryjoin' and 'secondaryjoin' expressions.

127.0.0.1 - - [26/Jul/2019 23:05:40] "GET /user HTTP/1.1" 500 -

The following is my program

from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy import Column, Integer, String, ForeignKey
from sqlalchemy.orm import relationship

Base = declarative_base()


class User(Base):

    __tablename__ = 'user'

    user_id = Column(Integer)
    passport_number = Column(String(8),  nullable=False, primary_key=True)
    user_email = Column(String(10), nullable=False)
    user_name = Column(String(10), nullable=False)

    car = relationship('Car', secondary='user')


class Car(Base):
    __tablename__ = 'car'

    car_number = Column(Integer, primary_key=True, nullable=False)
    car_model = Column(String(10), nullable=False)
    user_passport_number = Column(String(8), ForeignKey('user.passport_number'), primary_key=True)

    part = relationship('Part', secondary='car')


class Part(Base):

    __tablename__ = 'part'

    part_name = Column(String(10), primary_key=True, nullable=False)
    part_price = Column(Integer, nullable=False)
    car_number = Column(Integer, ForeignKey('car.car_number'), primary_key=True)

Answer 1

From the error message below:

...there are no foreign keys linking these tables via secondary table 'user'...

we understand that the error might be happening in the setting of the secondary table. Per the docs :

The default behavior of relationship() when constructing a join is that it equates the value of primary key columns on one side to that of foreign-key-referring columns on the other. We can change this criterion to be anything we'd like using the primaryjoin argument, as well as the secondaryjoin argument in the case when a “secondary” table is used.

Therefore, and assuming that you a have a one-to-many or one-to-one relationship between User and Car , you do not seem to need the secondary parameter:

class User(Base):

    __tablename__ = 'user'

    user_id = Column(Integer)
    passport_number = Column(String(8),  nullable=False, primary_key=True)
    user_email = Column(String(10), nullable=False)
    user_name = Column(String(10), nullable=False)

    car = relationship('Car', backref='User') #removed secondary relationship

Also, note that you are passing a primary_key constraint to the foreign key field, which is used only in one-to-one relationships, in case it was not intentional. In case it was, then the docs show a simple example for the one-to-one tables:

class User(Base):
    __tablename__ = 'user'
    passport_number = Column(String(8),  nullable=False, primary_key=True)
    car = relationship("Car", uselist=False, back_populates="car")

class Car(Base):
    __tablename__ = 'car'
    id = Column(Integer, primary_key=True)
    user_passport_number = Column(String(8), ForeignKey('user.passport_number'))
    user = relationship("User", back_populates="car")