N+1 query in user model

Question

I have an issue with a N+1 in my User model that I'm not sure how to fix. The method is setup like this:

 has_many :group_questions, through: :groups, source: :questions
 has_many :question_participants, as: :questionable
 has_many :questions, through: :question_participants

  # Collection of Users questions
  def all_questions
    group     = group_questions
    personal  = questions
    all_questions = group + personal
  end

it gathers questions related to a group that a User is in, but also personal questions directed to that person. Finally it merges them into one array.

The N+1's I'm getting are:

N+1 Query detected
  User => [:group_questions]
  Add to your finder: :includes => [:group_questions]

User => [:questions]
  Add to your finder: :includes => [:questions]

happens on this line:

current_user.all_questions.any?

Answer 1

class User 
  scope :with_questions, -> { includes(:questions, :group_questions) }

  def all_questions
    # No need for variables if you only use them once!
    group_questions + questions
  end
end

---

def show
  @user = User.with_questions.find(params[:id])
end

You could use default_scope but it's not really a good idea because it will slow down the cases where you don't need the joined records.

Added

I just saw your comment about getting the user from current_user . Although loading everything in one query would be optimal overriding the user loading in the authentication logic is a mess for a marginal performance gain. Instead you can do it like this:

@user = User.eager_load(:questions, :group_questions)
            .find(current_user.id)

eager_load will force ActiveRecord to load the records upfront in a single query. Try playing around with joins , includes and eager_load in the rails console and check the difference in the the resulting SQL queries.

• http://tomdallimore.com/blog/includes-vs-joins-in-rails-when-and-where/
• http://guides.rubyonrails.org/active_record_querying.html#joining-tables
• http://railscasts.com/episodes/22-eager-loading-revised?view=asciicast