Flask and SQLalchemy NoForeignKeysError: Could not determine join condition between parent/child tables on relationship User.posts [closed]

Question

I started out with the Mega Flask tutorial to build a simple blog style website. I've used this in correlation with some python studying to try and reinforce what I've learned. In an effort to learn even more I decided to switch out the tutorials deprecated OAuth login for a traditional flask-login. I'm encountering an issue though when I try to login.

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

With my limited understanding of Python, I've hit a pretty big roadblock here. I can't seem to figure out what I've done wrong.

Here's my models.py file.

followers = db.Table('followers',
            db.Column('follower_id', db.Integer, db.ForeignKey('user.id')),
            db.Column('followed_id', db.Integer, db.ForeignKey('user.id'))
            )

class User(db.Model):
    __tablename__ = "users"
    id = db.Column(db.Integer, primary_key=True)
    nickname = db.Column(db.String(64), index=True, unique=True)
    email = db.Column(db.String(120), index=True, unique=True)
    password = db.Column(db.String(100))
    posts = db.relationship('Post', backref='author', lazy='dynamic')
    about_me = db.Column(db.String(140))
    last_seen = db.Column(db.DateTime)
    followed = db.relationship('User',
                                secondary=followers,
                                primaryjoin=(followers.c.follower_id == id),
                                secondaryjoin=(followers.c.followed_id == id),
                                backref=db.backref('followers', lazy='dynamic'),
                                lazy='dynamic')

    #reviews = db.relationship('Review', backref='author', lazy='dynamic') This is the review connectino for the user. 

    @staticmethod
    def make_unique_nickname(nickname):
        if User.query.filter_by(nickname=nickname).first() is None:
            return nickname
        version = 2
        while True:
            new_nickname = nickname + str(version)
            if User.query.filter_by(nickname=new_nickname).first() is None:
                break
            version += 1
        return new_nickname

    def is_authenticated(self):
        return True

    def is_active(self):
        return True

    def is_anonymous(self):
        return False

    def get_id(self):
        try:
            return unicode(self.id) #python 2
        except:
            return str(self.id) #python 3

    def follow(self, user):
        if not self.is_following(user):
            self.followed.append(user)
            return self

    def unfollow(self, user):
        if self.is_following(user):
            self.followed.remove(user)
            return self

    def is_following(self, user):
        return self.followed.filter(
            followers.c.followed_id == user.id).count() > 0

    def followed_posts(self):
        return Post.query.join(
            followers, (followers.c.followed_id == Post.user_id)).filter(
                followers.c.follower_id == self.id).order_by(
                    Post.timestamp.desc())

    def __repr__(self):
            return '<User {}>'.format(self.nickname)

class Post(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    body = db.Column(db.String(140))
    timestamp = db.Column(db.DateTime)
    user_id = db.Column(db.Integer, db.ForeignKey('user.id'))

    def __repr__(self):
        return '<Post {}>'.format(self.body)

and here's my views.py file that should contain relevant code:

#This loads the user from the database
@lm.user_loader
def load_user(id):
    return User.query.get(int(id))


@app.before_request
def before_request():
    g.user = current_user
    if g.user.is_authenticated:
        g.user.last_seen = datetime.utcnow()
        db.session.add(g.user)
        db.session.commit()


@app.errorhandler(404)
def not_found_error(error):
    return render_template('404.html'), 404


@app.errorhandler(500)
def internal_error(error):
    db.session.rollback()
    return render_template('500.html'), 500


@app.route('/')
@app.route('/index')
@login_required
def index():
    user = g.user
    posts = [ #fake array of posts
        {
        'author': {'nickname': 'Zach'},
        'body': 'Reviewed this product!'
        },
        {
        'author': {'nickname': 'Mark'},
        'body': 'I like buttcheese!'
        }
    ]
    return render_template('index.html',
                            title='Home',
                            user=user,
                            posts=posts)



@app.route('/login', methods=['GET', 'POST'])
def login():
    if g.user is not None and g.user.is_authenticated:
        return redirect(url_for('index'))
    form = LoginForm()
    if form.validate_on_submit():
        user = User.query.get(form.nickname.data)
        session['remember_me'] = form.remember_me.data
        if user:
            if check_password_hash(user.password, form.password.data):
                user.authenticated = True
                db.session.add(user)
                db.session.commit()
                login_user(user, remember=True)
                flash("you've been logged in!, 'success'")
                return redirect(url_for('index'))
            else:
                flash('your email or password doesnt match!', 'error')
    return render_template('login.html', 
                            title='Sign In', 
                            form=form)


@app.route('/logout')
def logout():
    logout_user()
    return redirect(url_for('index'))

I'm really struggling with this because I don't understand DB relationships well enough yet. It's been a great learning experience, but if someone could help out that would be great.

Answer 1

I think this is a sqlalchemy error. When you define a relationship between User and Posts here:

posts = db.relationship('Post', backref='author', lazy='dynamic')

sqlalchemy needs to find a foreignkey on the database between user and posts. If you don't want to update your database schema you can define, like what you did on followed, a join that sqlalchemy can do to guess what are the posts related to the user.

In your case it might be:

posts = db.relationship('Post', backref='author', lazy='dynamic',
                        primaryjoin="User.id == Post.user_id")

Here is the documentation about it:

http://docs.sqlalchemy.org/en/latest/orm/join_conditions.html#specifying-alternate-join-conditions

Update:

I see that in fact Post.user_id has a foreign key defined. Maybe the sql server didn't create the foreign key? In any case what I said above may be a workaround.

Answer 2

Along with the post above I believe I have resolved the issue. I noticed that my tablename was actually "users" instead of "user". As a result, I needed to change all my foreign keys from user.id to users.id as it was looking for a table called "user" which didn't exist.