Migrations have always been considered one of the many killer features in Rails. And in Rails 3.1 Migrations got a new trick up their sleeve that will greatly simplify the process of maintaining both the
down logic. If you need a little refresher on what migrations are then I suggest reading the official Rails guide.
Lets start by looking at how a typical migration looks like in Rails 3.0
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This migration creates a
posts table with two fields —
body of type string and text, respectively. The
timestamps helper creates
datetime fields —
updated_at for free. To reverse this migration we simply need to drop the
posts table. The class method
down does precisely this. When Rails is applying a migration it runs the class method
up. To reverse the migration (as can be done with
rake db:rollback) it runs the class method
Two questions come up when you look at this migration —
- Why class methods instead of plain ‘old instance methods?
- For simple cases why not just define the up migration and have Rails take care of reversing the migration?
The ever awesome Aaron Patterson thought the same thing and decided to simplify things for you and I.
If you run the following command in Edge Rails, the genarated migration will look something like the example below:
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By default Rails 3.1 will generate migrations for models using the
change method that will hold the
up logic. When a rollback is requested Rails will figure out how to reverse the migration for you merely by examining the ‘up’ direction directives. Go ahead and apply the migration and then rollback. You should see something like the following:
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Notice how Rails has figured out that in order to reverse the migration, it needs to drop the newly created table.
What about commands that can’t be reversed?
There are certain commands like
remove_column that cannot be automatically reversed. This is because the information required to re-create the column is not available in the
remove_column command. If Rails encounters such commands while reversing a migration, an
ActiveRecord::IrreversibleMigration exception will be raised.
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If you try rolling back the above migration you will get something like:
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If you want to handle such cases manually you can still define the
down methods almost like before.
up and down instance methods
The only change to the old
down methods is that they are now instance methods. Say good bye to those awkward
self.down method definitions.
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More magic? :(
If you’re wondering how migration reversal is determined, and in the spirit of Jose Valim’s wish to see all Rails magic deconstructed, I thought I’d give a brief idea as to how Rails is reversing a migration automagically.
The magic, erm I mean heavy lifting, happens in the ActiveRecord::Migration::CommandRecorder class. Basically if you define a
change method in your migration and are applying the migration then the commands are executed as normal.
However while reversing the migration, the commands are recorded and a list of inverse commands is generated and run. Inverse commands are simply commands that perform the opposite of the original command. For eg: the inverse of
rename_table(old, new) is
rename_table(new, old). The logic to obtain an inverse of a command is provided in the class itself. For those commands whose inverse cannot be obtained,
ActiveRecord::IrreversibleMigration is raised.
That was a very simple overview of what is happening behind the scenes. I encourage you to take a look at the code for yourself to understand how it works.