Ruby on Rails

Hello, So I'm adding a Postgres Enum Type with a migration but I'm having a problem with db/schema.rb output breaking when I do this:

def up
  execute <<-SQL.squish
     CREATE TYPE three_letters_type AS ENUM (
       'A',
       'B',
       'C',
       );
  SQL
  execute <<-SQL.squish
     ALTER TABLE test
     ADD COLUMN three_letters three_letters_type;
  SQL
 end

I end up with the following comments in db/schema.rb -

# Could not dump table "test" because of following StandardError
#   Unknown type 'three_letters_type' for column 'three_letters'

It's quite important for me to be able to use Enums because unique-indexes across several columns are expensive and it's highly preferable not to use t.string (varchar) here. Would be great to hear if there is a work-around for this, thanks.