Include position in ranking for each challenge in user page

Additionally to showing the best player score and the best score for the
challenge, include the actual position of the best user score in the
ranking for that particular challenge.

Benchmarking: Created db:setup with challenges=300 users=10000, patched
db/seeds.db to have most challenges have a random number of entries in
the range 100..2,000, but 2% of them in the range 5,000..40,000, to
provide a more realist test scenario. (Creating all challenges with
20,000 entries just made all queries too expensive from the start.)

With that setup, I measured the effects of loading a user page, for a
user with entries in 36 challenges, which included all 6 challenges with
over 5,000 entries. More specifically, the 6 top challenges ranged from
17,829 to 37,251 entries.

I then loaded this page 10 times, with and without the changes in this PR.
I took the average and standard deviation of the 10 measurements for
each situation.

  - Without this PR:  0.25547s ± 0.00663s
  - With this PR:     2.23918s ± 0.05312s

This is a roughly 9x increase, which is perhaps quite expensive to make
it acceptable, even though the feature itself is very nice.

app/repositories/repository_challenge.rb

@@ -348,11 +348,14 @@ def self.player_best_scores(player_id)
         }
       },
       { "$unwind": "$entries" },
-      { "$match": { "entries.user_id": player_id } },
-      { "$sort": { "entries.score": 1 } },
+      { "$sort": { "entries.score": 1, "entries.created_at": 1 } },
       {
         "$group": {
-          "_id": '$_id',
+          "_id": {
+            "challenge_id": '$_id',
+            "user_id": '$entries.user_id'
+          },
+          "user_id": { "$first": '$entries.user_id'},
           "title": { "$first": '$title'},
           "description": { "$first": '$description'},
           "created_at": { "$first": '$created_at'},
@@ -362,7 +365,29 @@ def self.player_best_scores(player_id)
           "attempts": { "$sum":  1 }
         },
       },
-      { "$sort": { "created_at": -1 } },
+      { "$sort": { "best_player_score": 1, "created_at": 1 } },
+      {
+        "$group": {
+          "_id": "$_id.challenge_id",
+          "items": { "$push": "$$ROOT" },
+        }
+      },
+      { "$unwind": { "path": "$items", "includeArrayIndex": "items.position" }},
+      { "$match": { "items.user_id": player_id }},
+      { "$sort": { "items.created_at": -1 } },
+      {
+        "$project": {
+          "_id": 1,
+          "title": "$items.title",
+          "description": "$items.description",
+          "created_at": "$items.created_at",
+          "count_entries": "$items.count_entries",
+          "best_score": "$items.best_score",
+          "best_player_score": "$items.best_player_score",
+          "attempts": "$items.attempts",
+          "position": { "$add": ["$items.position", 1]}
+        }
+      },
     )
   end
 

app/views/users/show.erb

@@ -7,6 +7,7 @@
             <ul>
               <li>Best score: <b><%= player_challenge['best_score'] %></b></li>
               <li>Best player score: <b><%= player_challenge['best_player_score'] %></b></li>
+              <li>Position: <b>#<%= player_challenge['position'] %></b></li>
               <li>Number of attempts: <b><%= player_challenge['attempts'] %></b></li>
             </ul>
         </div>

spec/repositories/repository_challenge_spec.rb

@@ -430,6 +430,7 @@
         expect(challenge['best_score']).to eq(10)
         expect(challenge['best_player_score']).to eq(12)
         expect(challenge['attempts']).to eq(2)
+        expect(challenge['position']).to eq(2)
       end
     end
 
@@ -459,6 +460,7 @@
         expect(challenge['best_score']).to eq(14)
         expect(challenge['best_player_score']).to eq(14)
         expect(challenge['attempts']).to eq(1)
+        expect(challenge['position']).to eq(1)
 
         # And challenge1 is next.
         challenge = result[1]
@@ -469,6 +471,7 @@
         expect(challenge['best_score']).to eq(10)
         expect(challenge['best_player_score']).to eq(12)
         expect(challenge['attempts']).to eq(2)
+        expect(challenge['position']).to eq(2)
       end
     end