Flask-SQLAlchemy 'NoForeignKeysError'

Question

I am working on a Flask app, using Flask-SQLAlchemy extension for database interactions. Since I have multiple apps writing on the same DB, I was getting concurrency issues with SQLite and I wanted to switch to PostgreSQL instead. I am able to create the tables on new database without a problem and pgAdmin displays the tables and columns.

# working
def createTables():
    with app.app_context():
        from models import User, Invoice
        db.create_all()

But when it comes to adding a user, I am now getting an error: sqlalchemy.exc.NoForeignKeysError Although, I think, I declared one-to-many relationship in my models, based on the documentation , I get an error states that "there are no foreign keys linking these tables."

# not working
def create_test_user():
    with app.app_context():
    user = User(
        username="Bob",
        email="bob@email.com",
        password="test"
        )
    db.session.add(user) 
    db.session.commit()

The full error message:

""" NoForeignKeysError: Could not determine join condition between parent/child tables on relationship User.invoices 
- there are no foreign keys linking these tables.  
Ensure that referencing columns are associated with a ForeignKey or ForeignKeyConstraint, or specify a 'primaryjoin' expression. """

I can't figure out what causes the error. What is missing with my models?

# models.py
class User(db.Model):
    __tablename__ = "user"
    id = db.Column(db.Integer, primary_key=True)
    username = db.Column(db.String(20), unique=True, nullable=False)
    email = db.Column(db.String(120), unique=True, nullable=False)
    password = db.Column(db.String(60), nullable=False)
    invoices = db.relationship('Invoice', backref='user', lazy=True)

class Invoice(db.Model):
    __tablename__ = "invoice"
    id = db.Column(db.Integer, primary_key=True)
    user_id = db.Column(db.Integer, db.ForeignKey('user.id'), nullable=False)
    amount = db.Column(db.Integer, nullable=False)

Solved

Answer 1

Your code works for me. Maybe you need to re-create your tables or something similar. To be sure that we have the identical code: I have tested the following code:

class User(db.Model):
  __tablename__ = "user"
  id = db.Column(db.Integer, primary_key=True)
  username = db.Column(db.String(20), unique=True, nullable=False)
  email = db.Column(db.String(120), unique=True, nullable=False)
  password = db.Column(db.String(60), nullable=False)
  invoices = db.relationship('Invoice', backref='user', lazy=True)

class Invoice(db.Model):
  __tablename__ = "invoice"
  id = db.Column(db.Integer, primary_key=True)
  user_id = db.Column(db.Integer, db.ForeignKey('user.id'), nullable=False)
  amount = db.Column(db.Integer, nullable=False)

In the route:

user = User(
    username="Bob",
    email="bob@email.com",
    password="test"
    )
db.session.add(user) 
db.session.commit()

print(user)

Answer 2

I finally solved the problem and it was not where I was looking for. I was getting NoForeignKeysError due to importing a wrong model file during initializing the app. One of my imported modules was calling a wrong/old version of the model. It was causing the table relationship in the actual model to break I guess.

When I went through step by step create_test_user() I noticed that the error occurs actually during the class creation, before even it hits to db.session.add and I replicated the error even without a DB. I went through all my modules that are calling the models and caught wrong model import.