Database Migrations in Python with Flask with Alembic by Python Code Nemesis Code Like A Girl


Database Migrations in Python with Flask with Alembic by Python Code Nemesis Code Like A Girl

Note that the application script must be set in the FLASK_APP environment variable for all the above commands to work, as required by the flask command. The db upgrade command would first run migration 945fc , followed by migration b0c1f3d3617c. First, though, we need to add the details of the staging and production databases to the file. Now you’ll notice in your “versions” folder there is a migration file. You could generate (or edit) this file yourself; however, for most cases the auto-generated file will do. Note that the id fields were automatically set to 1 and 2 when those users were added.

Note that I never used/invoke db.create_all() before in the fresh container nor in development. Let’s say that for the next release of your application you have to introduce a change to your models, for example a new table needs to be added. Without migrations, you would need to figure out how to change the schema of your database, both in your development machine and then again in your server, and this could be a lot of work. Remember that the flask command relies on the FLASK_APP environment variable to know where the Flask application lives. For this application, you want to set FLASK_APP to the value, as discussed in Chapter 1. If you included a .flaskenv file in your project, then the all sub-commands of the flask command will automatically have access to the application.

Database Migrations with Alembic

All these files should be treated as part of your project from now on, and in particular, should be added to source control along with your application code. In the previous step, you upgraded your initial database version and added a price column to its products table. To demonstrate how to restore a previous state when managing database migrations, you will downgrade your current database and remove the price column from the products table. Next, we initialize our database repository using flask db init command.

  • The nice thing about SQLAlchemy is that it is an ORM not for one, but for many relational databases.
  • ORMs allow applications to manage a database using high-level entities such as classes, objects and methods instead of tables and SQL.
  • These files allow us to version our database and have it in source control.
  • We started by setting up Flask and SQLAlchemy, and defining a User model.

SQLAlchemy uses a so.mapped_column() function call assigned to each column to provide this additional configuration. In the case of id above, the column is configured as the primary key. For string columns many databases require a length to be given, so this is also included. I have included other optional arguments that allow me to indicate which fields are unique and indexed, which is important so that database is consistent and searches are efficient. The data that will be stored in the database will be represented by a collection of classes, usually called database models.

Written by Python Code Nemesis

In general, you will want to work with UTC dates and times in a server application instead of the local time where you are located. This ensures that you are using uniform timestamps regardless of where the users and the server are located. These timestamps will be converted to the user’s local time when they are displayed. The model class created in the previous section defines the initial database structure (or schema) for this application. But as the application continues to grow, it is likely that I will need to make changes to that structure such as adding new things, and sometimes to modify or remove items. Alembic (the migration framework used by Flask-Migrate) will make these schema changes in a way that does not require the database to be recreated from scratch every time a change is made.

flask migration

The application context that was pushed above allows Flask-SQLAlchemy to access the Flask application instance app without having to receive it as an argument. The extension looks in the app.config dictionary for the SQLALCHEMY_DATABASE_URI entry, which contains the database URL. Changes to a database are done in the context of a database session, which can flask developer be accessed as db.session. Multiple changes can be accumulated in a session and once all the changes have been registered you can issue a single db.session.commit(), which writes all the changes atomically. If at any time while working on a session there is an error, a call to db.session.rollback() will abort the session and remove any changes stored in it.

Flask-Migrate 4.0.5

Inside of “migrations” you will see that it has a folder called “versions”, which will contain the migration scripts as they are created. Finally, we added the db command to the manager so that we can run the migrations from the command line. This was by no means a perfect solution, but it at least showed how database migrations can be run when deploying Flask applications. Each time the source code changes or new database migrations are created, a new container can be deployed that automatically applies database changes and runs our application.

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