Rails的3.1独特的发现和缺少的属性 [英] Rails 3.1 distinct find and missing attributes

问题描述

class State < ActiveRecord::Base
  has_many :cities
end

class City < ActiveRecord::Base
  belongs_to :state
  has_many :companies
end

class Company < ActiveRecord::Base
  belongs_to :city
end

我想列出所有的状态，各自的城市，包含至少有一家公司注册的。我的第一次尝试是下面的查询：

I'm trying to list all states, and their respective cities, that contain at least one company registered. My first try was the following query:

states = State.joins(:cities => :companies).includes(:cities)

这工作，但我最终得到重复，如果一个国家有多个城市设有公司，在它。然后我改变了查询：

Which works, but I end up getting duplicates if a state has more than one city with companies in it. I then changed the query to:

states = State.joins(:cities => :companies).includes(:cities).select("distinct(states.id)")

这个查询几乎工程。我有机会到城市（州[0] .cities），而且没有重复，但如果我尝试从国家访问对象的属性，我收到以下错误：

This query almost works. I have access to the cities (states[0].cities), and there are no duplicates, but if I try to access an attribute from the State object, I get the following error:

ruby-1.9.2-p290 :056 >states[0].name
ActiveModel::MissingAttributeError: missing attribute: name

我该如何解决这个问题？

How can I solve this?

在此先感谢

推荐答案

SELECT语句覆盖默认的（ SELECT * FROM ... 变成 SELECT DISTINCT（state.id）FROM ... ），这样的结果并不包括你的状态表（其中属性是从推断）的列。试着改变你的选择的方法如下：

Your select statement overrides the default (SELECT * FROM ... becomes SELECT distinct(state.id) FROM...) so the results don't include the columns of your state table (where the attributes are inferred from). Try changing your select method to the following:

.select("distinct(states.id), states.*")

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