Add Downs

A plugin for semi-automating the creation of migration 'down' methods.

David A. Black
April 2008

====

This plugin adds a rake task, rake db:migrations:add_downs, which automatically
writes down methods for your migrations. It does this by overriding a bunch of
migration commands (add_column, rename_column, etc.). It then runs all your
'up' methods. The overridden commands figure out (as best they can) what you
want the down equivalent to be, and print it out. 

The results are stored in RAILS_ROOT/tmp/migrations. You can, and should,
examine them to make sure they're OK. You might see "UNKNOWN COLUMN TYPE!" in
one or more instructions in the down method; that's because add_downs couldn't
figure out, just from the migration file, what column type to add or change to.
You have to fill those in by hand. 

Only migration files that already have a down method are processed. That will
include any that are created automatically by Rails when you generate either a
model or a migration. If the migration just contains the canonical "drop_table"
down method, it will be processed and the method will simply be reconstituted. 

add_downs makes some pretty naive and non-heuristic assumptions about migration
file naming -- namely, that it can chop off three characters at the beginning
and end, and derive the correct class name from what's left. If you've got any
non-vanilla naming going on, be warned. Nothing in db/migrate gets overwritten,
though, so the worst that should happen is that it doesn't produce the right
results. (But please back everything up first anyway!)

Example
=======

If you start with this, in db/migrate/001_add_downs_example.rb:

class AddDownsExample < ActiveRecord::Migration
  def self.up
    create_table :examples do |t|
      t.string :text
    end
    add_column :examples, :number, :integer
    add_index :examples, :number
    rename_column :examples, :text, :body
    change_column :examples, :number, :float
    remove_column :examples, :text
  end

  def down
  end
end

and run this command:

  rake db:migrate:add_downs

you'll end up with this in tmp/migrations/001_add_downs_example.rb:

class AddDownsExample < ActiveRecord::Migration
  def self.up
    create_table :examples do |t|
      t.string :text
    end
    add_column :examples, :number, :integer
    add_index :examples, :number
    rename_column :examples, :text, :body
    change_column :examples, :number, :float
    remove_column :examples, :text
  end

  def down
    add_column :examples, :text, UNKNOWN COLUMN TYPE!
    change_column :examples, :number, UNKNOWN COLUMN TYPE!
    rename_column :examples, :body, :text
    remove_index :examples, :column => :number
    remove_column :examples, :number
    drop_table :examples
  end
end

That's about it. Comments and corrections welcome. 

Copyright (c) 2008, David A. Black (dblack@rubypal.com),
released under the MIT license