Some kind of automatic adding index on forgotten migrations

I use rails 3.2 and by default it does add indexes if column type is specified as references.

i type: rails g migration AddFieldToTable field:references

result:

class AddFieldToTable < ActiveRecord::Migration

def change

add_column :tables, :field, :references

end

end

there is no migrations there or i type wrong, on pgSQL, rails 3.2