The head alias always points to the most recent migration, which in our case is the first and only one currently in the repository. To understand the problem you have to think about how Alembic (the migration flask developer engine used by Flask-Migrate) generates database migrations. The contents of a migration are obtained by running a comparison between the current model definitions and the current schema of your database.

This command effectively restores the tables that were downgraded above. Since database migrations do not preserve the data stored in the database, downgrading and then upgrading has the effect of quickly emptying all the tables. The first command tells Flask-Migrate to apply the database migrations in reverse order. When the downgrade command is not given a target, it downgrades one revision. The base target causes all migrations to be downgraded, until the database is left at its initial state, with no tables. The migration script needs to be reviewed and edited, as Alembic is not always able to detect every change you make to your models.

Flask-Migrate 4.0.5

This is a good way of avoiding circular dependencies, since these objects are often used in multiple places when it comes to Flask applications. When I run myapp.py it runs OK, but the tables are not created automatically (I found that first migration has to be done). Myapp.py contains all config files and server init code with all other functionality such as that for home page and sign up page. We created templates using Jinja2 and HTML to display the users in a table and allow users to add new users.

In particular, Alembic is currently unable to detect table name changes, column name changes, or anonymously named constraints. A detailed summary of limitations can be found in the Alembic autogenerate documentation. Once finalized, the migration script also needs to be added to version control. When the container started, our script automatically ran alembic migrations in order to create the needed database tables. In addition, our Flask application was able to create a post and save it to the database. Now you should find a new migration file under migrations/versions/.

Databases in Flask

Calling the all() method of the results object converts the results to a plain list. Are you wondering how do all these database operations know what database to use? 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. I have made you suffer through a long process to define the database, but I haven’t shown you how everything works yet.

• The flask db migrate command does not make any changes to the database, it just generates the migration script.
• This is a good practice that I recommend you follow as well, because with this variable it is easy to redirect to another database.
• This allows Alembic to migrate the database to any point in the history, even to older versions, by using the downgrade path.
• The User class has a new posts field, that is initialized with so.relationship().

Notice, that in addition to revision ID, alembic also keeps track of down_revision ID. This way alembic is able to upgrade and downgrade database migrations in the correct order. The trick to generate this initial migration for a project that uses an up to date database is to temporarily switch to an empty database, just for the flask db migrate command. In my projects I use a DATABASE_URL environment variable to configure the location of the database.

How to do first migration in flask?

The User class created above will represent users stored in the database. The class inherits from db.Model, a base class for all models from Flask-SQLAlchemy. These are the columns that will be created in the corresponding database table.

• This is a good way of avoiding circular dependencies, since these objects are often used in multiple places when it comes to Flask applications.
• Each time a change is made to the database schema, a migration script is added to the repository with the details of the change.
• The class inherits from db.Model, a base class for all models from Flask-SQLAlchemy.
• Changes to a database are done in the context of a database session, which can be accessed as db.session.
• 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.