postgresql - What's the proper way to add an index to a table in a Rails migration? -

i'm using rails 5 postgres 9.5 . have following migration

class createcryptoindexcurrencies < activerecord::migration[5.0]   def change     create_table :crypto_index_currencies |t|       t.references :crypto_currency, foreign_key: true       t.date :join_date, :null => false, :default => time.now       t.timestamps     end     add_index :crypto_index_currencies, :crypto_currency, unique: true   end end 

upon running migration, dying error

pg::undefinedcolumn: error:  column "crypto_currency" not exist 

what proper way add index? table name want reference called "crypto_currencies".

 add_index 'crypto_index_currencies', ['crypto_currency'], name: "index_crypto_index_currencies_on_crypto_currency", unique: true, using: :btree 

using: :btree optional.