Memoize a user's personal projects count

The method User#projects_limit_left would run "personal_projects.count"
but such a query is not memoized. As a result multiple calls to
User#projects_limit_left would result in multiple COUNT(*) queries being
executed.

To work around this this commit adds User#personal_projects_count which
simply memoizes the result of the COUNT(*) in an instance variable.

app/models/user.rb

@@ -632,7 +632,11 @@ class User < ActiveRecord::Base
   end
 
   def projects_limit_left
-    projects_limit - personal_projects.count
+    projects_limit - personal_projects_count
+  end
+
+  def personal_projects_count
+    @personal_projects_count ||= personal_projects.count
   end
 
   def projects_limit_percent

changelogs/unreleased/memoize-user-personal-projects-count.yml

+---
+title: Memoize the number of personal projects a user has to reduce COUNT queries
+merge_request:
+author:

spec/models/user_spec.rb

@@ -1976,4 +1976,28 @@ describe User do
       expect(user.allow_password_authentication?).to be_falsey
     end
   end
+
+  describe '#personal_projects_count' do
+    it 'returns the number of personal projects using a single query' do
+      user = build(:user)
+      projects = double(:projects, count: 1)
+
+      expect(user).to receive(:personal_projects).once.and_return(projects)
+
+      2.times do
+        expect(user.personal_projects_count).to eq(1)
+      end
+    end
+  end
+
+  describe '#projects_limit_left' do
+    it 'returns the number of projects that can be created by the user' do
+      user = build(:user)
+
+      allow(user).to receive(:projects_limit).and_return(10)
+      allow(user).to receive(:personal_projects_count).and_return(5)
+
+      expect(user.projects_limit_left).to eq(5)
+    end
+  end
 end