Add logic for ranking the gems by download

app/models/rubygem.rb

@@ -183,6 +183,15 @@ def downloads
     gem_download&.count || 0
   end
 
+  def gem_rank
+    gem_ranking = update_gems_ranking
+
+    return gem_ranking[id] if gem_ranking.key?(id)
+
+    Rails.cache.delete("gems_ranking")
+    update_gems_ranking[id]
+  end
+
   def most_recent_version
     versions.most_recent
   end
@@ -446,4 +455,17 @@ def bulk_reorder_versions
     sanitized_query = ActiveRecord::Base.send(:sanitize_sql_array, update_query)
     ActiveRecord::Base.connection.execute(sanitized_query)
   end
+
+  def update_gems_ranking
+    Rails.cache.fetch("gems_ranking", expires_in: 24.hours) do
+      update_query = <<-SQL.squish
+        SELECT rubygem_id, row_number() OVER(ORDER BY count DESC) AS position
+        FROM gem_downloads
+        WHERE version_id = 0 AND rubygem_id <> 0
+      SQL
+      sanitized_query = ActiveRecord::Base.send(:sanitize_sql_array, [update_query])
+      pg_result = ActiveRecord::Base.connection.execute(sanitized_query)
+      pg_result.to_h { |row| [row["rubygem_id"], row["position"]] }
+    end
+  end
 end

test/models/rubygem_test.rb

@@ -1102,4 +1102,41 @@ class RubygemTest < ActiveSupport::TestCase
       refute_predicate @version_three, :yanked?
     end
   end
+
+  context "#gem_rank" do
+    setup do
+      @rank_one = create(:rubygem, downloads: 5)
+      @rank_two = create(:rubygem, downloads: 1)
+
+      @dummy_rank = { @rank_one.id => 1, @rank_two.id => 2 }
+    end
+
+    should "write to cache" do
+      @rank_one.gem_rank
+
+      assert_equal Rails.cache.read("gems_ranking"), @dummy_rank
+    end
+
+    should "read from cache" do
+      @rank_two.gem_rank
+
+      GemDownload.increment(10, rubygem_id: @rank_two.id)
+
+      @rank_two.gem_rank
+
+      assert_equal Rails.cache.read("gems_ranking"), @dummy_rank
+    end
+
+    should "return the gem rank" do
+      assert_equal 1, @rank_one.gem_rank
+    end
+
+    should "update when rubygem not ranked" do
+      @rank_two.gem_rank
+
+      @rank_three = create(:rubygem, downloads: 3)
+
+      assert_equal(2, @rank_three.gem_rank)
+    end
+  end
 end