I got this error when I run my code.

“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:

models.py

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’ {self.id}: {self.body}>’

routes.py

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)

__init__.py

import flask
import os
from flask_sqlalchemy import SQLAlchemy
basedir = os.path.abspath(os.path.dirname(__file__))
#instance of the flask class
app_obj = flask.Flask(__name__)
app_obj.config.from_mapping(
SECRET_KEY = ‘it-dont-matter’,
SQLALCHEMY_DATABASE_URI = ‘sqlite:///’ os.path.join(basedir, ‘app.db’),
SQLALCHEMY_TRACK_MODIFICATIONS = False
)
db = SQLAlchemy(app_obj)
from app import routes, models

forms.py

from flask_wtf import FlaskForm
from wtforms import StringField, IntegerField, BooleanField, SubmitField
from wtforms.validators import DataRequired
class TopCities(FlaskForm):
city_name = StringField(‘City Name’,validators=[DataRequired()])
city_rank = IntegerField(‘City Rank’,validators=[DataRequired()])
is_visited = BooleanField(‘Visited’)
submit = SubmitField(‘Submit’)

base.html

{% if title %}

{% else %}

{% endif %}

{% with messages = get_flashed_messages() %}
{% if messages %}

{% for m in message %}
• {{ m }}

{% endfor %}

{% endif %}
{% endwith %}
{% block content %}
{% endblock %}

home.html

{% extends “base.html” %}

{% block content %}

Welcomee {{name}}, here are top destinations!

goooogle

{% for city in top_cities %}
• {{ city }}

{% endfor %}

{{ form.hidden_tag() }}

{{ form.city_name.label }}

{{ form.city_name(size=40) }}

{{ form.city_rank.label }}

{{ form.city_rank(size=40) }}

{{ form.is_visited.label }}

{{ form.is_visited() }}

{{ ‘Submit’ form.submit() }}