#### Migrations and Rollbacks

In Rails applications, for database related changes like creating a table, adding a column, removing a column, etc. we write a lot of migrations.

To run those migrations, we use the db:migrate command.

> rails db:migrate


But sometimes, while writing migrations we might end up making some mistakes. To revert such mistakes we use db:rollback, which restores the database to a state, before the latest migration was run.

> rails db:rollback


We can also pass STEP as an environment variable to this command, to specify the number of migrations to revert. For example, if we have to revert latest 3 migrations then we’ll run the following command.

> rails db:rollback STEP=3


#### Multiple databases

Rails 6 has added support for multiple databases.

Thus we will have multiple directories similar to migrate under db folder. We can pass an additional db option while creating the new migration and this will add that migration under a particular database’s migrate directory.

> rails g migration AddUser first_name:string last_name:string --db=secondary


We can run all database migrations in one go by running the following command.

> rails db:migrate


If we want to run migrations of the specific database then we can run the following command.

> rails db:migrate:secondary


Here secondary is the name of the database.

Now if we want to revert the latest migration then we can run the following command.

> rails db:rollback


Since we didn’t have a way to specify which database we really want to rollback the migration from, it always ended up reverting the latest migration of the primary database.

To fix this issue, Rails 6.0.2.1 has added support for database name in the db:rollback command similar to db:migrate.

> rails db:rollback:[NAME]


Now, if we run db:rollback command without name for multiple database applications then it throws the following error.

> rails db:rollback
rails aborted!
You're using a multiple database application. To use db:rollback you must run the namespaced task with a VERSION. Available tasks are db:rollback:primary and db:rollback:secondary.


To fix this, we will need to run the following command.

> rails db:rollback:secondary


The above command will revert the latest migration of the secondary database.