Flask SQLAlchemy 2 ForeignKey columns going back to the same table - database

Question

I have a Users table and a Tasks table. The Tasks table has an "author" column and an "assignee" column, both of which are foreign keys to the unique ID in Users.
As you might expect, this produced an error: "Could not determine join condition between parent/child tables on relationship Users.created_tasks - there are multiple foreign key paths linking the tables."
What's another way to record both the "author" and "assignee" if I wanted to have both columns in the table?
class Tasks(db.Model):
// ...
author = db.Column(db.Integer, db.ForeignKey('users.id'))
assignee = db.Column(db.Integer, db.ForeignKey('users.id'))
class Users(db.Model, UserMixin):
// ...
created_tasks = db.relationship('Tasks', backref = 'ctasks_user')
assigned_tasks = db.relationship('Tasks', backref = 'atasks_user')

Answer 1

You can specify the primaryjoin attribute of each relationship to differentiate the two:
import sqlalchemy as sa
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import relationship, sessionmaker
connection_uri = (
"mssql+pyodbc://#localhost:49242/myDb?driver=ODBC+Driver+17+for+SQL+Server"
)
engine = sa.create_engine(connection_uri)
with engine.begin() as conn:
conn.execute(sa.text("DROP TABLE IF EXISTS task_t"))
conn.execute(sa.text("DROP TABLE IF EXISTS user_t"))
Base = declarative_base()
class Task(Base):
__tablename__ = "task_t"
id = sa.Column(sa.Integer, primary_key=True)
description = sa.Column(sa.String(50))
author = sa.Column(sa.Integer, sa.ForeignKey("user_t.id"))
assignee = sa.Column(sa.Integer, sa.ForeignKey("user_t.id"))
def __repr__(self):
return f"<Task(id={self.id}, description='{self.description}')>"
class User(Base):
__tablename__ = "user_t"
id = sa.Column(sa.Integer, primary_key=True)
name = sa.Column(sa.String(50))
created_tasks = relationship(
Task,
primaryjoin="User.id == Task.author",
backref="user_created_tasks",
)
assigned_tasks = relationship(
Task,
primaryjoin="User.id == Task.assignee",
backref="user_assigned_tasks",
)
def __repr__(self):
return f"<User(id={self.id}, name='{self.name}')>"
Base.metadata.create_all(engine)
Session = sessionmaker(bind=engine)
session = Session()
homer = User(name="Homer")
bart = User(name="Bart")
lisa = User(name="Lisa")
session.add_all([homer, bart, lisa])
session.commit()
mow_the_lawn = Task(
description="Mow the lawn", author=homer.id, assignee=bart.id
)
wash_the_car = Task(
description="Wash the car", author=homer.id, assignee=lisa.id
)
session.add_all([mow_the_lawn, wash_the_car])
session.commit()
with engine.begin() as conn:
result = conn.execute(
sa.text("SELECT * FROM user_t ORDER BY id")
).fetchall()
print(result)
# [(1, 'Homer'), (2, 'Bart'), (3, 'Lisa')]
result = conn.execute(
sa.text("SELECT * FROM task_t ORDER BY id")
).fetchall()
print(result)
# [(1, 'Mow the lawn', 1, 2), (2, 'Wash the car', 1, 3)]
print(homer.created_tasks)
# [ <Task(id=1, description='Mow the lawn')>, <Task(id=2, description='Wash the car')>]
print(bart.assigned_tasks)
# [ <Task(id=1, description='Mow the lawn')>]