flask migrate seemed to delete all my database data

Question

Here is my migration file :

from flask import Flask
from flask_sqlalchemy import SQLAlchemy
from flask_script import Manager
from flask_migrate import Migrate, MigrateCommand
import models

app = Flask(__name__)
app.config['SQLALCHEMY_DATABASE_URI'] = 'sqlite:///' + 'C:\\flaskDB\\commBorn3.db'

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

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



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

I ran the standard commands :

python app.py db init

python app.py db migrate

python app.py db upgrade

The resulting database was a file of the same name with about the same size in kb but only one table, "alembic_version" which had nothing in it. I tried to downgrade and found all my tables returned but they were empty. What did I do wrong? Where is all the data hiding (same kb size of file)?

Answer 1

You should always check the migration file in migrations/versions created after db migrate, and execute db upgrade only when everything looks alright.

The problem is that in models.py you import a db = SQLAlchemy(app) instance from your "normal" app, not from the migration script. So in the migration script you actually don't define any model, and this is why it deletes all your tables in the database.

The solution is easy: just use the migration script's app context when you import the models:

from flask import Flask
from flask_sqlalchemy import SQLAlchemy
from flask_script import Manager
from flask_migrate import Migrate, MigrateCommand

app = Flask(__name__)
app.config['SQLALCHEMY_DATABASE_URI'] = 'sqlite:///' + 'C:\\flaskDB\\commBorn3.db'

db = SQLAlchemy(app)

# Import database models with app context
with app.app_context():
  from models import *

migrate = Migrate(app, db)

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

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