Question

In my application Users register for Events, which belong to a Stream. The registrations are managed in the Registration model, which have a boolean field called ‘attended‘.

I’m trying to generate a leaderboard and need to know: the total number of registrations for each user, as well as a count for user registrations in each individual event stream.

I’m trying this (in User.rb):

# returns an array of users and their attendence count
def self.attendance_counts
    User.all( 
      :select => "users.*, sum(attended) as attendance_count", 
      :joins => 'left join `registrations` ON registrations.user_id = users.id', 
      :group => 'registrations.user_id', 
      :order => 'attendance_count DESC'
    )
end

The generated SQL works for just returning the total attended count for each user when I run it in the database, but all that gets returned is the User record in Rails.

I’m about to give up and hardcode a counter_cache for each stream (they are fairly fixed) into the User table, which gets manually updated whenever the attended attribute changes on a Registration model save.

Still, I’m really curious as to how to perform a query like this. It must come up all the time when calculating statistics and reports on records with relationships.

Your time and consideration is much appreciated. Thanks in advance.

Answer 1

Firstly as a couple of points on style and rails functions to help you with building DB queries.

1) You’re better writing this as a scope rather than a method i.e.

scope attendance_counts, select("users.*, sum(attended) as attendance_count").joins(:registrations).group('registrations.user_id').order('attendance_count DESC')

2) It’s better not to call all/find/first on the query you’ve built up until you actually need it (i.e. in the controller or view). That way if you decide to implement action / fragment caching later on the DB query won’t get called if the cached action / fragment is served to the user.

3) Rails has a series of functions to help with aggregating db data. for example if you only wanted a user’s id and the sum of attended you could use something like the following code:

Registrations.group(:user_id).sum(:attended)

Other functions include count, avg, minimum, maximum

Finally in answer to your question, rails will create an attribute for you to access the value of any custom fields you have in the select part of your query. e.g.

@users = User.attendance_counts
@users[0].attendance_count # The attendance count for the first user returned by the query