Add primary key to schema_migrations table

Question

I would like to run the following migration to my Rails(4.x) app but would like to get some feedback from the Rails community first.

class AddUniqueIndexToSchemaMigrations < ActiveRecord::Migration
  def change
    add_column :schema_migrations, :id, :primary_key
    add_index :schema_migrations, ["version"], :name => "unique_versions", :unique => true
  end
end

The database in use is MySQL (5.x). Adding this index allows me execute SQL such as:

INSERT INTO schema_migrations 
  (version) 
VALUES ( '2014xxxxxx' ) 
  ON DUPLICATE KEY 
  UPDATE foo=bar; // or some other bit of cleverness

Is there any reason why this is going off the rails? I don't want to create a situation where I'm fighting against Rails internals.

Thanks in advance!