在Rails迁移中向表中添加索引的正确方法是什么？

Question

我正在使用Rails 5和PostGres 9.5。我有以下迁移

class CreateCryptoIndexCurrencies < ActiveRecord::Migration[5.0] 
    def change 
    create_table :crypto_index_currencies do |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 

在运行迁移，这是死亡与此错误

PG::UndefinedColumn: ERROR: column "crypto_currency" does not exist 

什么是添加索引的正确方法？我想引用的表名称为“crypto_currencies”。

Answer 1

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

内的语法：：B树其可选的。

Answer 2

这是添加它使用CREATE_TABLE块

class CreateCryptoIndexCurrencies < ActiveRecord::Migration[5.0] 
    def change 
    create_table :crypto_index_currencies do |t| 
     t.references :crypto_currency, foreign_key: true 
     t.date :join_date, :null => false, :default => Time.now 
     t.index :crypto_currency, unique: true 
    end 
    end 
end