database migration in flask - sqlalchemy

Question

flask-restplus and sqlalchemy to design my backend. I am using flask-migrate library so that I can adjust my database tables when there is any such requirements.

Here is my directory structure

-mainfolder/
     -app/
         -v1/
           -app.py
           -apis/
              -__init__.py
              -some folders.
               .
               .
               .
           -database/
              -__init__.py
              -models.py
           -config/
               -__init__.py
               -detect_config_file.py
            -

     -wsgi.py

The code in my app.py is

import logging.config
import json
import os
import sys
from flask import Flask, Blueprint
from flask_restplus import Api
from flask_jwt_extended import JWTManager
from flask_cors import CORS
from database import *
from config.detect_config_file import config

app = Flask(__name__)
cors = CORS(app)
api = Api(version='1.0', title='Plug-n-Play APIs',
      description='Demostration of APIs behind plug-n-play dashboard')
logging_conf_path = 
 os.path.normpath(os.path.join(os.path.dirname(__file__), '../../logging.conf'))
 logging.config.fileConfig(logging_conf_path)
 log = logging.getLogger(__name__)

def configure_app(flask_app):

     flask_app.config['JWT_SECRET_KEY'] = config['JWT_SECRET_KEY']
     flask_app.config['JWT_BLACKLIST_ENABLED'] = config['JWT_BLACKLIST_ENABLED']
     flask_app.config['JWT_BLACKLIST_TOKEN_CHECKS'] = config['JWT_BLACKLIST_TOKEN_CHECKS']
     flask_app.config['SQLALCHEMY_DATABASE_URI'] = 'mysql+pymysql://'+ config['DB_USERNAME'] + ':' + config['DB_PASSWORD']+'@'+ config['DB_HOST'] + '/' + config['DB_NAME']
     flask_app.config['SQLALCHEMY_TRACK_MODIFICATIONS'] = config['DB_TRACK_MODIFICATIONS']


def initialize_app(flask_app):
   configure_app(flask_app)

    blueprint = Blueprint('api', __name__, url_prefix='/api/v1')
    api.init_app(blueprint)

    flask_app.register_blueprint(blueprint)

    db.init_app(flask_app)

    with flask_app.app_context():
      initDB()

    jwt = JWTManager(flask_app)


def main():
  initialize_app(app)
  log.info('>>>>> Starting development server <<<<<')
  app.run(debug=config['APP_DEBUG'], host="0.0.0.0")


if __name__ == "__main__":
  main()

models.py file in database folder is

from database import db
#importing from __init__.py of database folder

from sqlalchemy.types import JSON
from sqlalchemy_utils.types.choice import ChoiceType
from config.detect_config_file import config
from flask_sqlalchemy import SQLAlchemy
from flask_script import Manager
from flask_migrate import Migrate, MigrateCommand
from app import app
#importing the flask app from app.py 

 db = SQLAlchemy(app)
 migrate = Migrate(app, db)

 manager = Manager(app)
 manager.add_command('db', MigrateCommand)

class UserModel(db.Model):
    __tablename__ = 'users'

    id = db.Column(db.Integer, primary_key = True)
    username = db.Column(db.String(120), unique = True, nullable = False)

if __name__ == '__main__':
    manager.run()

the __init__.py file in the database folder is

from flask_sqlalchemy import SQLAlchemy

db = SQLAlchemy()


def initDB():
    from database.models import *
    db.create_all()

My thought process here is that when i run app.py then it will create the tables in my database(i have already created the database in mysql) the segment of code in app.py

with flask_app.app_context():
   initDB()

when the schema changes then in that case my I will run these commands to update my databases without having to drop my database and create again.

$python models.py db init
$python models.py db migrate
$python models.py db upgrade

But this is not working. I am getting a circular dependency error when i try to import app from app in the models.py

Please have a look into my code and tell me if my approach is correct or there is a better way to do this. My main aim is to achieve the database migrations just like we have in django so that I can change my database schema easily.

First problem you need to solve is that you have two database instances, one in models.py and another in the __init__.py of your database package. You should use a single database instance. The problem with the circular imports is the standard problem that occurs with Flask, you can address it by moving the database imports below the point where app is created. — Miguel Grinberg
– Miguel Grinberg, Commented May 17, 2018 at 22:29

Jessi · Accepted Answer · 2018-07-09 15:09:33Z

I'd suggest instantiating both app/db in app/init.py, so that app/db can be imported from a single place. After you've instantiate your manager, import the manager in run.py which is outside of the app directory and switch it between app.run()/ manager.run()

model.py

from app import db
class UserModel(db.Model):
    __tablename__ = 'users'

    id = db.Column(db.Integer, primary_key = True)
    username = db.Column(db.String(120), unique = True, nullable = False)

project/app/init.py

def create_app():
    app = Flask(__name__)
    return app

app = create_app()
# import this db to your model.
db = SQLAlchemy(app)
manager = Manager(app)
manager.add_command('db', MigrateCommand)

project/run.py

# Placed from app import manager right above main block.
from app import manager

if __name__ == '__main__':
    # switch between run/manager based on your need.
    #manager.run() # for DB migration
    app.run(threaded=True, debug=True)

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