How do you make remove_column reversible?

Simply adding the 3rd argument (the column’s :type) to the remove_column method makes that migration reversible. So the OP’s original code actually did work, as in:

remove_column :foos, :bar, :boolean

The rest of this answer was an attempt to discover why this method would not have been working, but the OP ended up getting it to work.

I see somewhat contrary info in the documentation for ActiveRecord::Migration:

Some commands like remove_column cannot be reversed. If you care to define how to move up and down in these cases, you should define the up and down methods as before.

For a list of commands that are reversible, please see ActiveRecord::Migration::CommandRecorder.

And this from ActiveRecord::Migration::CommandRecorder:

ActiveRecord::Migration::CommandRecorder records commands done during a migration and knows how to reverse those commands. The CommandRecorder knows how to invert the following commands:

add_column

add_index

add_timestamps

create_table

create_join_table

remove_timestamps

rename_column

rename_index

rename_table

Anyway, it appears that this documentation is out of date… Digging into the source on github:

The method that’s giving you grief is:

def invert_remove_column(args)
  raise ActiveRecord::IrreversibleMigration, "remove_column is only reversible if given a type." if args.size <= 2
  super
end

I gave this a shot… setup a migration on my Rails 4.1.2 app and the migration worked both ways — up and down. Here was my migration:

class TestRemoveColumn < ActiveRecord::Migration
  def change
    remove_column :contacts, :test, :boolean
  end
end

I also tried with the :boolean argument missing and got the same error as you’re talking about. Are you sure you’re on the final version of Rails 4.1.2 — not one of the release candidates? If you are, I’d suggest putting a binding.pry into the Rails source for the invert_remove_column method to inspect the arguments list and see what’s going on. To do so, just run bundle open activerecord and then explore to: lib/active_record/migration/command_recorder.rb:128.