Rails 作用域未返回正确的数据

Question

当我使用我在模型中准备的作用域时，rails 返回不正确的数据。

我的模型：

class CurrencyRate < ActiveRecord::Base
scope :eur_today, -> {where(currency: "eur").where(date: Time.now.in_time_zone.to_date).first}
end

来自 rails 控制台的信息：

2.3.0 :011 > CurrencyRate.eur_today
CurrencyRate Load (0.2ms)  SELECT  "currency_rates".* FROM 
"currency_rates" WHERE "currency_rates"."currency" = ? AND 
"currency_rates"."date" = ?  ORDER BY "currency_rates"."id" ASC LIMIT 1  
[["currency", "eur"], ["date", "2017-08-09"]]
CurrencyRate Load (0.2ms)  SELECT "currency_rates".* FROM 
"currency_rates"
=> #<ActiveRecord::Relation [#<CurrencyRate id: 2, currency: "eur", 
sale: 4.248546249999998, purchase: 4.265333125, date: "2017-08-08", 
created_at: "2017-08-08 20:52:08", updated_at: "2017-08-08 20:54:10", 
sale_percentage_diff: nil, purchase_percentage_diff: nil>]>

当我使用与范围相同的查询时，返回的数据是正确的：

2.3.0 :012 > CurrencyRate.where(currency: "eur").where(date: Time.now.in_time_zone.to_date).first 
CurrencyRate Load (0.2ms)  SELECT  "currency_rates".* FROM 
"currency_rates" WHERE "currency_rates"."currency" = ? AND 
"currency_rates"."date" = ?  ORDER BY "currency_rates"."id" ASC LIMIT 1  
[["currency", "eur"], ["date", "2017-08-09"]]
=> nil 

为什么范围不能正常工作？

Answer 1

因为作用域必须返回ActiveRecord::Relation。删除作用域中的first调用，并在作用域外使用它。为什么？因为作用域必须是可链接的。