Rails: How to add add_index to existing table

Question

I already migrated a table called units with several columns. I was wondering how to migrate in a stand alone \'add_index\' to this table using the cmd. Is this code correct:

Answer 1

To remove an index, you must use remove_index with the same table and column specification as the self.up's add_index has. So:

def self.down
  remove_index :units, :lesson_id
end

A multi-column index example would be:

def self.down
  remove_index :units, [:lesson_id, :user_id]
end