Rails 3 ActiveRecord & Postgres: order by count on association

问题

I have the following 2 rails models:

class Profile < ActiveRecord::Base
  belongs_to :user

  has_many :votes, through: :user

  default_scope includes(:user)

end

and

class Vote < ActiveRecord::Base
  attr_accessible :by, :for

  belongs_to :by, class_name: "User"
  belongs_to :for, class_name: "User"

  validates :by, :for, presence: true

  validates_uniqueness_of(:by, scope: :for)
end

I'm trying to create a "top" scope on Profile which sorts the Profiles based on the number of "for" votes the associated user record has received

A vote is created by a user, for a user. The "by" column indicates the user who casted the vote, the "for" column indicates the user that has received a vote. I'm trying to get the profile of the user that has received the most votes.

This is what I've got so far:

   scope :top,
    select("profile.*, count(votes.id) AS votes_count").
    joins(:votes, :user).
    order("votes_count DESC")

This does not work with the following error:

ActiveRecord::StatementInvalid: PG::Error: ERROR: column "votes_count" does not exist

It also doesn't take the "for" column into account

Can anyone help me out ?

回答1:

You're missing a grouping, I believe. Here's the query you're trying to do:

select profiles.*, count(votes.id) as votes_count 
  from profiles 
  left join votes on votes.for_id = profiles.user_id 
  group by profiles.id 
  order by votes_count desc;

So, let's turn this into an ActiveRecord scope:

scope :top, joins('left join votes on votes.for_id = profiles.user_id').
  select('profiles.*, count(votes.id) as votes_count').
  group('profiles.id').
  order('votes_count desc')

回答2:

ActiveRecord way

Profile.joins(:votes).group("profiles.id").order("count(profiles.id) DESC")

来源：https://stackoverflow.com/questions/15478909/rails-3-activerecord-postgres-order-by-count-on-association