Writing Migrations

Reversible Alembic database migrations are created by generating scripts with explicit upgrade and downgrade paths for adding, modifying, or removing tables, columns, and indexes. This ensures safe schema evolution by allowing forward and backward execution.

Data migrations in Python should be kept distinct from schema modifications to ensure safe database evolution. You generate separate Alembic scripts to populate existing data, such as filling a new column, after applying the structural schema changes.

The best way to add a new column to a SQL table while supporting a downgrade is to generate an Alembic migration script. This script defines the upgrade function to add the column and the downgrade function to safely remove it.

Yes, you can test forward and backward execution of database schema migrations. The process emphasizes testing both the upgrade and downgrade paths in your Alembic scripts to verify that schema changes can be safely applied and reversed.

Yes, this approach supports modifying indexes and constraints in an existing database schema. Alembic migration scripts can be generated to handle adding, modifying, or removing these database objects alongside standard table and column alterations.

You need to separate data migrations from schema modifications to ensure safe database schema evolution. Keeping them distinct prevents data loss during reversible upgrades and downgrades, allowing structural changes and data population to be managed independently.