ruby-on-rails - 如何在查询结果中添加虚拟字段

Question

为了提高性能，我写了以下内容:

a = Address.joins(:vacancies).select('"addresses"."city", COUNT (*) AS total_count').group('"addresses"."city"').order('total_count DESC')

代替:

a = Address.all.keep_if{ |a| a.vacancies.all_active.size > 0 }.uniq{ |a| a.city }.map{ |a| [a.city, a.vacancies.all_active.size] }.sort{ |aa, ab| aa[1] <=> ab[1] }.reverse

但是，total_count 字段不会显示在结果中。如果我在 sqlite 中运行查询，它就会显示出来。我向 Address 模型添加了一个虚拟属性，但这没有帮助。如何解决这个问题？

此外，这是我的 Rails 控制台输出:

1.9.3p194 :010 > a = Address.joins(:vacancies).select("addresses.city, count(*) as total_count").group("addresses.city").order("total_count DESC")
  Address Load (3.9ms)  SELECT addresses.city, count(*) as total_count FROM "addresses" INNER JOIN "vacancies" ON "vacancies"."address_id" = "addresses"."id" GROUP BY addresses.city ORDER BY total_count DESC
 => [#<Address city: "Amsterdam">, #<Address city: "Rotterdam">, #<Address city: "Utrecht">, #<Address city: "Den haag">, #<Address city: "Delft">, #<Address city: "Hilversum">, #<Address city: "Amstelveen">, #<Address city: "Rijswijk">, #<Address city: "Breda">, #<Address city: "Zwolle">, #<Address city: "Tilburg">, #<Address city: "Amersfoort">, #<Address city: "Amsterdam centrum">, #<Address city: "Eindhoven">, #<Address city: "Next nature netwerk, amsterdam">, #<Address city: "Almere">, #<Address city: "Ameide">, #<Address city: "Arnhem">, #<Address city: "Baarlo">, #<Address city: "Capelle a/d ijssel">, #<Address city: "De lier">, #<Address city: "Leiden">, #<Address city: "Reigerstraat 16, breda">, #<Address city: "Rottedam">, #<Address city: "'s-hertogenbosch ">, #<Address city: "Alkmaar">, #<Address city: "Alphen aan den rijn">, #<Address city: "Amsterdam singel 137">, #<Address city: "Amsterdam, keizergracht">, #<Address city: "Barendrecht">, #<Address city: "Den haag e.o.">, #<Address city: "Deventer">, #<Address city: "Dordrecht">, #<Address city: "Groningen">, #<Address city: "Gummo, amsterdam">, #<Address city: "Hillversum">, #<Address city: "Houten">, #<Address city: "Lisse">, #<Address city: "Moti, breda">, #<Address city: "Nijmegen">, #<Address city: "Rotterdam ">, #<Address city: "Tuil (waardenburg)">, #<Address city: "Zaltbommel">] 
1.9.3p194 :011 > a.first.total_count
 => nil 
1.9.3p194 :012 > a.first.to_json
 => "{\"city\":\"Amsterdam\",\"total_count\":null}"

Answer 1

首先，删除虚拟属性(我假设你的意思是你的模型中有 attr_accessor :total_count)。

Active Record 的 inspect 方法(即在控制台中生成输出的方法)被硬连接为仅显示有关它期望的属性的信息，即与表中实际列相对应的信息。仅因为您向 select 子句添加内容而存在的属性将不会显示。他们还在那里 - 你可以做

a.first.total_count

或

a.first.attributes

去看看。在您的控制台 session 中，a.first.total_count 不起作用，因为您的 attr_accessor 隐藏了数据库属性。

to_json 方法的相似之处在于默认情况下它会序列化它知道的属性。您可以通过执行以下操作在输出中包含 total_count

a.to_json(:methods => [:total_count])