SQLAlchemy primaryjoin 为一对多关系返回空 object

Question

I have tables defined as:

class Car(Base):
    __tablename__ = 'cars'
    CarID = Column('id', Integer, primary_key=True)
    CarDetails = relationship(
          "CarDetails", 
          back_populates="Car", 
          primaryjoin="and_(CarDetails.CarID == Car.CarID, CarDetails.CarNumber == 1)",
        )
    CustomCarDetails = relationship(
              "CarDetails", back_populates="Car", uselist=True)

class CarDetails(Base):
    __tablename__ = 'car_details'
    ID = Column('id', Integer, primary_key=True)
    CarNumber = Column('car_number', Integer, default=1)
    CarID = Column('table_id', Integer, ForeignKey(
        'tables.id'), nullable=False)
    Car = relationship("Car", back_populates="CustomCarDetails")

Car: CarDetails follows 1-to-many relation. In CarDetails column (in Car), I want to populate entries with CarNumber=1 for the respective Car. With the above definition, I get empty object. I have referred this also. What am I missing here?

When I have it defined as follows, CarDetails returns a random entry from CarDetails(may return the record with CarNumber != 1 ), whereas I only want entry corresponding to CarNumber=1 :

CarDetails = relationship(
                "CarDetails", 
                back_populates="Car", 
                uselist=False)

How can I fix this?

Answer 1

Adding uselist=False worked here:

CarDetails = relationship(
      "CarDetails", 
      back_populates="Car", 
      primaryjoin="and_(CarDetails.CarID == Car.CarID, CarDetails.CarNumber == 1)",
      uselist=False
    )

I couldn't find this in the documentation!!