Rails 3.1 distinct find and missing attributes

问题

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

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)

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)")

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

How can I solve this?

Thanks in advance

回答1:

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.*")

来源：https://stackoverflow.com/questions/7419577/rails-3-1-distinct-find-and-missing-attributes