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The usefulness of db migration rollback

Many people talk about db migration, especially the possibility of rollback.

I doubt how useful this is at all, because the db and model schema is closely related to application logic (MVC).

Suppose I rolled back some migration. So what? The application will not work, because its logic is completely dependent on db.

What are the rollback options for db migration?

Update 1

Main question

Why is rollback presented as a function when I need to change the code?

I do not create migrations, for example, add_another_field_to_table. Instead, each migration file fully describes each table in the database. When I need to change something in my database, I just modify the migration file, but it doesn’t roll it back .

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