Remove N+1 queries from users autocomplete

Both of these were related to groups:

1. We need to preload routes (using the `with_route` scope) if we're
   going to get the group's path.
2. We were counting each group's members separately.

They're in the same commit because the spec for N+1 detection wouldn't
pass with only one of these fixes.

app/models/members/group_member.rb

@@ -14,6 +14,8 @@ class GroupMember < Member
 
   scope :in_groups, ->(groups) { where(source_id: groups.select(:id)) }
 
+  scope :count_users_by_group_id, -> { joins(:user).group(:source_id).count }
+
   after_create :update_two_factor_requirement, unless: :invite?
   after_destroy :update_two_factor_requirement, unless: :invite?
 

app/services/concerns/users/participable_service.rb

@@ -28,19 +28,35 @@ module Users
     end
 
     def groups
-      current_user.authorized_groups.sort_by(&:path).map do |group|
-        group_as_hash(group)
+      group_counts = GroupMember
+                       .in_groups(current_user.authorized_groups)
+                       .non_request
+                       .count_users_by_group_id
+
+      current_user.authorized_groups.with_route.sort_by(&:path).map do |group|
+        group_as_hash(group, group_counts)
       end
     end
 
     private
 
     def user_as_hash(user)
-      { type: user.class.name, username: user.username, name: user.name, avatar_url: user.avatar_url }
-    end
-
-    def group_as_hash(group)
-      { type: group.class.name, username: group.full_path, name: group.full_name, avatar_url: group.avatar_url, count: group.users.count }
+      {
+        type: user.class.name,
+        username: user.username,
+        name: user.name,
+        avatar_url: user.avatar_url
+      }
+    end
+
+    def group_as_hash(group, group_counts)
+      {
+        type: group.class.name,
+        username: group.full_path,
+        name: group.full_name,
+        avatar_url: group.avatar_url,
+        count: group_counts.fetch(group.id, 0)
+      }
     end
   end
 end

changelogs/unreleased/fix-routes-n-plus-one-in-user-autocomplete.yml

+---
+title: Fix some N+1s in loading routes and counting members for groups in @-autocomplete
+merge_request: 26491
+author:
+type: performance

spec/models/members/group_member_spec.rb

 require 'spec_helper'
 
 describe GroupMember do
+  describe '.count_users_by_group_id' do
+    it 'counts users by group ID' do
+      user_1 = create(:user)
+      user_2 = create(:user)
+      group_1 = create(:group)
+      group_2 = create(:group)
+
+      group_1.add_owner(user_1)
+      group_1.add_owner(user_2)
+      group_2.add_owner(user_1)
+
+      expect(described_class.count_users_by_group_id).to eq(group_1.id => 2,
+                                                            group_2.id => 1)
+    end
+  end
+
   describe '.access_level_roles' do
     it 'returns Gitlab::Access.options_with_owner' do
       expect(described_class.access_level_roles).to eq(Gitlab::Access.options_with_owner)

spec/services/projects/participants_service_spec.rb

@@ -2,29 +2,56 @@ require 'spec_helper'
 
 describe Projects::ParticipantsService do
   describe '#groups' do
+    set(:user) { create(:user) }
+    set(:project) { create(:project, :public) }
+    let(:service) { described_class.new(project, user) }
+
+    it 'avoids N+1 queries' do
+      group_1 = create(:group)
+      group_1.add_owner(user)
+
+      service.groups # Run general application warmup queries
+      control_count = ActiveRecord::QueryRecorder.new { service.groups }.count
+
+      group_2 = create(:group)
+      group_2.add_owner(user)
+
+      expect { service.groups }.not_to exceed_query_limit(control_count)
+    end
+
+    it 'returns correct user counts for groups' do
+      group_1 = create(:group)
+      group_1.add_owner(user)
+      group_1.add_owner(create(:user))
+
+      group_2 = create(:group)
+      group_2.add_owner(user)
+      create(:group_member, :access_request, group: group_2, user: create(:user))
+
+      expect(service.groups).to contain_exactly(
+        a_hash_including(name: group_1.full_name, count: 2),
+        a_hash_including(name: group_2.full_name, count: 1)
+      )
+    end
+
     describe 'avatar_url' do
-      let(:project) { create(:project, :public) }
       let(:group) { create(:group, avatar: fixture_file_upload('spec/fixtures/dk.png')) }
-      let(:user) { create(:user) }
-      let!(:group_member) { create(:group_member, group: group, user: user) }
 
-      it 'should return an url for the avatar' do
-        participants = described_class.new(project, user)
-        groups = participants.groups
+      before do
+        group.add_owner(user)
+      end
 
-        expect(groups.size).to eq 1
-        expect(groups.first[:avatar_url]).to eq("/uploads/-/system/group/avatar/#{group.id}/dk.png")
+      it 'should return an url for the avatar' do
+        expect(service.groups.size).to eq 1
+        expect(service.groups.first[:avatar_url]).to eq("/uploads/-/system/group/avatar/#{group.id}/dk.png")
       end
 
       it 'should return an url for the avatar with relative url' do
         stub_config_setting(relative_url_root: '/gitlab')
         stub_config_setting(url: Settings.send(:build_gitlab_url))
 
-        participants = described_class.new(project, user)
-        groups = participants.groups
-
-        expect(groups.size).to eq 1
-        expect(groups.first[:avatar_url]).to eq("/gitlab/uploads/-/system/group/avatar/#{group.id}/dk.png")
+        expect(service.groups.size).to eq 1
+        expect(service.groups.first[:avatar_url]).to eq("/gitlab/uploads/-/system/group/avatar/#{group.id}/dk.png")
       end
     end
   end