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Initialize the database with Flask-Migrate

Activate your virtual environment and run this command:

flask db init

This will create a migrations folder inside your project folder.

In the migrations folder you'll find a few things:

  • The versions folder is where migration scripts will be placed. These will be used by Alembic to make changes to the database.
  • alembic.ini is the Alembic configuration file.
  • env.py is a script used by Alembic to generate migration files.
  • script.py.mako is the template file for migration files.

Generate the first migration to set up the database​

Now that we're set up, we need to make sure that the database we want to use is currently empty. In our case, since we're using SQLite, just delete data.db.

Then, run this command:

flask db migrate

This will create the migration file.

caution

It's important to double-check the migration script and make sure it is correct! Compare it with your model definitions and make sure nothing is missing.

Now let's actually apply the migration:

flask db upgrade

This will create the data.db file. If you were using another RDBMS (like PostgreSQL or MySQL), this command would create the tables using the existing model definitions.

How does the database know which version it's on?

When using Alembic to create the database tables from scratch, it creates an extra table with a single row, that stores the current migration version.

You'll note in each migration script there is information about the previous migration and the next migration.

This is why it's important to never rename the migration files or change the revision identifiers at the top of those files.