Question

Given the following models:

team.rb

class Team < ActiveRecord::Base
    has_many :events, :dependent => :destroy
    has_many :challenges, :through => :events

    validates :name, :presence => true, :uniqueness => true
end

challenge.rb

class Challenge < ActiveRecord::Base
    has_many :events, :dependent => :destroy
    has_many :teams, :through => :events

    validates :name, :presence => true, :uniqueness => true
    validates :flag, :presence => true, :uniqueness => true
end

event.rb

class Event < ActiveRecord::Base
    belongs_to :team
    belongs_to :challenge

    validates :team,      :presence => true
    validates :challenge, :presence => true
end

I want to display the teams with the highest “rank”.
Where the highest ranking team has the most challenges completed (events).
If there is a tie at X events then the team that completed the Xth event
first has the highest rank.

So I can easily sort the teams based on the number of events and then show them.

Like so:

def index
    @teams = Team.includes(:events).
                  select("*, COUNT(events.id)").
                  group("teams.id, events.id").
                  order("COUNT(events.id) DESC")
end

However I don’t know how to handle the case where there is a tie.
Does anybody know a good way of doing this with SQL?

I would rather use SQL to do this versus performing an extra step
on the app server.

Thanks!!

Answer 1

So I got it working.
Thanks xnm. You sent me in the right direction. Can’t believe I forgot about MAX. 🙁

I also had to fix my GROUP BY…

Here is the working version:

class Team < ActiveRecord::Base
    ...

    def self.ranked
        all(:select => "teams.*, COUNT(events.id) AS challenges_completed, MAX(events.created_at) AS last_event",
            :joins => "LEFT OUTER JOIN events ON events.team_id = teams.id",
            :group => "teams.id, events.team_id",
            :order => "challenges_completed DESC, last_event ASC")
    end
end