n+1 problem

gistfile1.rb

class User < ActiveRecord::Base

  has_many :mails
  
  def messages_for_recipient(user)
    mails.where(user_id: user.id).map(&:messages).flatten
  end

end

class Mail < ActiveRecord::Base
  has_many :messages
end

class UserController < ApplicationController

  def index
    @users = User.includes(:messages)
  end

end

#index.html.haml
- @users.each do |user|
  = user.messages.count # дополнительного запроса нет
  = render partial: messages, collection: user.messages_for_recipient(user) # дополнительный sql запрос (problem!)
  
#_messages.html.haml
message.id
message.subject
message.body