thoughtbot’s dotfiles include an alias
mapping migrate to
rake db:migrate db:rollback && rake db:migrate
It is also a good practice in our workflow to use the migrate alias in place
of rake db:migrate. Let’s explore what this command does and why we recommend
it.
Reversibility
rake db:migrate db:rollback will run any pending migrations and then attempt
to roll back those migrations. If the migration and rollback are successful
(signified by &&), rake db:migrate will then run the
migration again. If the command is not successful, an error will be thrown. In
this way we can be confident our migrations’ down commands are syntactically
valid (no typos) and logically valid (no invalid castings of values, for example).
Beyond having a short alias in place of a multi-word rake command, ensuring that each migration we write is reversible is very handy for iterative development locally and for dreaded production rollbacks.
An Exception
One time when we break this guideline is when we explicitly require an
irreversible migration because the transformation is destructive.
In the special circumstances when this behavior is required, the
down command should raise an ActiveRecord::IrreversibleMigration exception
as stated by the
docs. As a
result, a rollback is impossible and we’ll run rake db:migrate instead of migrate.
What’s next
We have found having a frictionless way of running migrations in both directions has saved us lots of time and headaches.
If you’d like to learn more about handling irreversible migrations check out the following resources:
- ActiveRecord::Migration documention
- Rails Guides on Migrations
Happy migrating!
