Flask-SQLAlchemy- sqlalchemy.exc.ArgumentError: Mapper mapped class could not assemble any primary key columns for mapped table in existing db

Question

I am trying to build an app using Flask and a pre existing sqlite database which has 3 tables. I tried checking what the issue is by trying to import the DB in terminal with a minimal app.

As suggested in this tutorial I used the following code, where city is a table in the db:

from flask import Flask
from flask_sqlalchemy import SQLAlchemy

app = Flask(__name__)
app.config['SQLALCHEMY_DATABASE_URI'] = 'sqlite:////path/test.sqlite'
db = SQLAlchemy(app)


class city(db.Model):
     __tablename__ = 'city'

Following this method I was able to import the db and the table in terminal using the following command.

from yourapplication import db

However when I try to do the same with following method as suggested here in the same tutorial, since my project is getting a little bigger and it is the recommended method in the book I am following as well as Flask tutorial, I get the above mentioned error.

from flask import Flask
from flask_sqlalchemy import SQLAlchemy

SQLALCHEMY_DATABASE_URI = 'sqlite:////path/test.sqlite'

db = SQLAlchemy()

def create_app():
   app = Flask(__name__)
   db.init_app(app)
   return app


class city(db.Model):
    __tablename__ = 'city'

Start of the error message as it might be relevant:

from yourapplication import db
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/path/yourapplication.py", line 14, in <module>
  class city(db.Model):

Now I am not sure the second method is done in the right way in my code since documentation doesn't mention what to do to provide address of the database but I am getting the same error that I have been getting in my original project. My main app also works when all the views and code other than the templates are in the same file.

Since I can import the db in the first method, I assume that problem isn't with the DB and it has primary key. I also know that because the db works perfectly fine in first method and I created the db from a pandas dataframe using the following code:

df1.to_sql("city", conn, dtype={'pkc': 'INTEGER PRIMARY KEY'},    if_exists='replace', index=False)

where pkc has the same value as the index of df1.

This is the book I am following.