Question

I have a fairly straightforward query that I can’t seem to get right….

My model:

• User – has many Friendships (with other users)
• User – has many submissions
• User – has many comments
• User – has many votes

I need a count that represents

All current_user’s friends, whose submissions, comments or votes created_at dates are > current_user.last_refresh_date

Right now, I am building up an array by iterating over friendships and adding all submissions, comment and votes. I then re-iterate this built-up array while comparing the dates to determine if the count should be incremented. Not the most ideal solution.

Edit:

@TobiasCohen

Efficient solution. Thanks!

Followup:

I wish to add yet one more count to the present query. I need to count all new comments & votes on the current_user.submissions that are not part of the original count (ie. not a friend).

Psuedo-code :

current_user.submissions.join(:comments, :votes, :friends).where(‘last_activity >? AND friend_id != ?’, current_user.last_refresh_date, current_user.id).count

I can’t quite get the query correct (new to complex queries via active record).

I was going to make it a separate query and then add it to the original count. Can it be absorbed into one query instead of two?

Answer 1

I think you’d get the best results by adding a cache column on User, let’s call it :last_activity_at, then update this with an after_create callback on Submission, Comment and Vote.

class Submission < ActiveRecord::Base
  belongs_to :user

  after_create :update_user_last_activity_at

  private
    def update_user_last_activity_at
      user.update_attribute :last_activity_at, Time.now
    end
end

You could then fetch users simply with:

current_user.friends.where('last_activity_at > ?', current_user.last_refresh_date)