How do I fix sqlalchemy.exc.NoForeignKeysError?

I am writing a program that each time city is submitted it should display and let user know it was added. I also got this message when I run the program. "sqlalchemy.exc.NoForeignKeysError: Could not determine join condition between parent/child tables on relationship TopCities.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."

Here is my code:

from app import db
from datetime import datetime

class TopCities(db.Model):
    #create column for database
    #primary_key means it is unique
    id = db.Column(db.Integer, primary_key=True)
    city_name= db.Column(db.String(64), unique=True, index=True)
    city_rank = db.Column(db.Integer())
    posts = db.relationship('Post', backref='author', lazy='dynamic')

    def __repr__(self):
        return f'{self.city_name} with rank {self.city_rank}'

class Post(db.Model):
    #create column for database
    #primary_key means it is unique
    id = db.Column(db.Integer, primary_key=True)
    body = db.Column(db.String(256))
    timestamp = db.Column(db.DateTime, default=datetime.utcnow)
    user_id = db.Column(db.Integer, db.ForeignKey('TopCities.id'))

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



from app import app_obj
from app.forms import TopCities
from flask import render_template, flash, redirect
from app import db
from app.models import TopCities, Post

@app_obj.route("/login", methods = ['GET','POST'])
def login():
    name = 'Khuong'
    title = 'Top Cities'
    top_cities = TopCities.query.all()
    form = TopCities()
    if form.validate_on_submit():
    top_cities = Topcities.query.all(city_name=form.city_name.data)
        if top_cities is None or not top_cities.check_city_rank(form.city_rank.data):
            flask('Invalid city_name or city_rank')
        else:
            top_cities = TopCities(city_name = form.city_name.data, city_rank = form.city_rank.data)
            db.sesion.add(top_cities)
            db.session.commit()
            flask('New city is added!')
            return redirect('/')

    return render_template("home.html", name=name, title=title, form=form)