add index in migrate file

db/migrate/20170403174309_create_lookups.rb

@@ -1,7 +1,7 @@
 class CreateLookups < ActiveRecord::Migration[5.1]
   def change
     create_table :lookups do |t|
-      t.string :lookup_type, :null => false
+      t.string :lookup_type, :null => false, :index => true
       t.string :name, :null => false
       t.string :value, :null => false
     end