Question

The title is the problem in a nutshell. Here are the simplified models I am using :

class Test
  include Mongoid::Document

  field :name, :type => String
  embeds_many :questions
  attr_accessible :name, :questions      
end

class Question
  include Mongoid::Document

  field :text, :type => String

  embedded_in :test
  has_many :answers
  attr_accessible :text, :test, answers    
end

class Answer
  include Mongoid::Document

  field :value, :type => Integer

  belongs_to :question
  belongs_to :user
  attr_accessible :value, :question, :user      
end

class User
  include Mongoid::Document

  has_many :answers
  attr_accessible :answers    
end 

I want to be able to retrieve all the unanswered questions in a test for a user with the minimal amount of database queries.

This is the best solution I have thought up so far

answered = []
user.answers.each {|a| answered << a.question_id}
test.questions.where(:_id.nin => answered).length

Any help with this is much appreciated.

Answer 1

UPDATE:
you need to query for the non-existence of a constrained select:

test.questions.find_by_sql("WHERE NOT EXISTS (SELECT id FROM answers WHERE answers.question_id = questions.id AND answers.user_id = #{user.id}") 

or count 0 of a conditional join

test.questions.joins("INNER JOIN answers ON answers.question_id = questions.id AND answers.user_id = #{user.id} AS user_answers").where('COUNT(user_answers.id) = 0')

let me know if these work. any insight on their performance?

ORIGINAL ANSWER, INCORRECT cos it was assuming we need totally unanswered questions ASKED by the user:

use a counter cache on answers. This assumes an answers_count integer column on the questions db table.

class Answer
  include Mongoid::Document

  field :value, :type => Integer

  belongs_to :question, :counter_cache => true
  belongs_to :user
  attr_accessible :value, :question, :user      
end

Given this, your query can be:

test.questions.where(answers_count: 0, user_id: user)