scope for count children has_many relation - sql

Question

I have two models
# == Schema Information
#
# Table name: answers
#
# id :integer not null, primary key
# content :text
# question_id :integer
# accept :boolean
# created_at :datetime not null
# updated_at :datetime not null
# user_id :integer
#
class Answer < ActiveRecord::Base
attr_accessible :accept, :content, :question_id, :user
belongs_to :question
belongs_to :user
delegate :username, to: :user, allow_nil: true, prefix: 'owner'
end
and Question
# == Schema Information
#
# Table name: questions
#
# id :integer not null, primary key
# title :string(255)
# content :text default(""), not null
# created_at :datetime not null
# updated_at :datetime not null
# user_id :integer
# viewed_count :integer default(0)
#
class Question < ActiveRecord::Base
validates_presence_of :title, :content, :user
attr_accessible :content, :title, :tag_list
acts_as_taggable
belongs_to :user, :counter_cache => true
has_many :answers
delegate :username, to: :user, allow_nil: true, prefix: 'owner'
scope :owner, joins(:user)
scope :without_answer, joins(:answers).
select('questions.id').
group('questions.id').
having('count(answers.id) = 0')
validate :validation_of_tag_list
def self.no_answer
Question.all.select{|question|question.answers.count == 0}
end
The scope without_answer and class method no_answer theoretically should be same. However, I run them in the console as below:
Loading development environment (Rails 3.2.13)
irb(main):001:0> Question.without_answer
Question Load (0.6ms) SELECT questions.id FROM `questions` INNER JOIN `answers` ON `answers`.`question_id` = `questions`.`id` GROUP BY questions.id HAVING count(answers.id) = 0
=> []
irb(main):002:0> Question.no_answer
Question Load (0.6ms) SELECT `questions`.* FROM `questions`
(0.5ms) SELECT COUNT(*) FROM `answers` WHERE `answers`.`question_id` = 1
(0.4ms) SELECT COUNT(*) FROM `answers` WHERE `answers`.`question_id` = 2
(0.4ms) SELECT COUNT(*) FROM `answers` WHERE `answers`.`question_id` = 16
(0.4ms) SELECT COUNT(*) FROM `answers` WHERE `answers`.`question_id` = 17
(0.3ms) SELECT COUNT(*) FROM `answers` WHERE `answers`.`question_id` = 34
=> [#<Question id: 2, title: "Here is the second", content: "here you go\r\n", created_at: "2013-04-20 00:34:15", updated_at: "2013-04-20 00:34:15", user_id: nil, viewed_count: 0>, #<Question id: 16, title: "my question", content: "Here is my question", created_at: "2013-04-21 02:02:47", updated_at: "2013-04-23 02:29:27", user_id: 1, viewed_count: 1>, #<Question id: 17, title: "Hello", content: "me", created_at: "2013-04-23 00:37:56", updated_at: "2013-04-23 00:37:56", user_id: nil, viewed_count: 0>, #<Question id: 34, title: "Question title", content: "question content", created_at: "2013-04-23 04:57:49", updated_at: "2013-04-23 04:57:49", user_id: 42, viewed_count: 0>]
why the scope dose not work as expect?
which way will be better to due with such situation or even better solution?

Answer 1

Your without_answer scope is very close, but needs an outer join like this:
scope :without_answer,
joins('LEFT OUTER JOIN answers ON answers.question_id = questions.id').
select('questions.id').
group('questions.id').
having('count(answers.id) = 0')
Then, you can get the count with length:
Question.without_answer.length
Note: if you want without_answer to be the same as no_answer (i.e. return actual Question objects), you would need to remove the select.
A simpler and faster way to count the unanswered questions is like this:
Question.joins('LEFT OUTER JOIN answers ON answers.question_id = questions.id').
where('answers.id' => nil).count
Also, this will return the same as no_answer as-is, simply use all instead of count.