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I'm having trouble with Many-to-One relationships between my SQLAlchemy models. The relationship between ChangeOrder (many) and Contract (one) is fine, but the one between LineItem (many) and ChangeOrder (one) isn't working.

I've tried both approaches suggested in the basic relationships docs and both fail for the LineItem to ChangeOrder relationship.

# using back_populates
class Contract(Base):
 __tablename__ = "contract"
 id = Column(Integer, primary_key=True)
 change_orders = relationship("ChangeOrder", back_populates="contract")


class ChangeOrder(Base):
 __tablename__ = "changeorder"
 id = Column(Integer, primary_key=True)
 line_items = relationship("LineItem", back_populates="change_order")
 contract_id = Column(Integer, ForeignKey("contract.id"))
 contract = relationship("Contract", back_populates="change_orders")


class LineItem(Base):
 __tablename__ = "lineitem"
 id = Column(Integer, primary_key=True)
 change_order_id = Column(Integer, ForeignKey("changeorder.id"))
 change_order = relationship("ChangeOrder", back_populates="line_items")


def test_insert_change_order(db_session, item):
 c = Contract()
 db_session.add(c)
 db_session.commit()
 co = ChangeOrder(contract_id=c.id)
 db_session.add(co)
 db_session.commit()
 row = db_session.query(Contract).get(c.id)
 assert len(row.change_orders) == 1 # this Many-to-One works
 li = LineItem(change_order_id=co.id)
 db_session.add(li)
 db_session.commit()
 row = db_session.query(ChangeOrder).get(co.id)
 assert len(row.line_items) == 1 # this Many-to-One does not

I also tried the backref approach, but it has the same problem.

# using backref
class LineItem(Base):
 __tablename__ = "lineitem"
 id = Column(Integer, primary_key=True)
 change_order_id = Column(Integer, ForeignKey("changeorder.id"))
 change_order = relationship("ChangeOrder", backref="line_items")


class ChangeOrder(Base):
 __tablename__ = "changeorder"
 id = Column(Integer, primary_key=True)
 contract_id = Column(Integer, ForeignKey("contract.id"))
 contract = relationship("Contract", backref="change_orders")


class Contract(Base):
 __tablename__ = "contract"
 id = Column(Integer, primary_key=True)

conftest.py

import pytest
from flask_sqlalchemy import SQLAlchemy

from frontend.app import app


@pytest.fixture
def testapp():
 db = SQLAlchemy()
 app.config["SQLALCHEMY_ECHO"] = True
 with app.app_context():
 db.create_all()
 yield app
 db.session.remove()
 db.drop_all()


@pytest.fixture(scope="session")
def database():
 db = SQLAlchemy()
 with app.app_context():
 db.create_all()
 yield db


@pytest.fixture(scope="session")
def _db(database):
 return database