Optimize finding a membership for a user to avoid extra queries

app/models/user.rb

@@ -1073,6 +1073,15 @@ class User < ActiveRecord::Base
     super
   end
 
+  # Get the membership for an object without loading the source of the membership
+  #
+  # This loads the entire `members` relation of the user, used on `current_user`
+  def membership_for_object(object)
+    members.sort('access_level_asc').detect do |member|
+      object.is_a?(member.source_type.constantize) && member.source_id == object.id
+    end
+  end
+
   protected
 
   # override, from Devise::Validatable

app/serializers/group_child_entity.rb

@@ -40,9 +40,7 @@ class GroupChildEntity < Grape::Entity
   end
 
   def permission
-    return unless request&.current_user
-
-    request.current_user.members.find_by(source: object)&.human_access
+    membership&.human_access
   end
 
   # Project only attributes
@@ -55,11 +53,11 @@ class GroupChildEntity < Grape::Entity
          unless: lambda { |_instance, _options| project? }
 
   def leave_path
-    leave_group_group_members_path(object)
+    leave_group_members_path(object)
   end
 
   def can_leave
-    if membership = object.members_and_requesters.find_by(user: request.current_user)
+    if membership
       can?(request.current_user, :destroy_group_member, membership)
     else
       false
@@ -73,4 +71,12 @@ class GroupChildEntity < Grape::Entity
   def number_users_with_delimiter
     number_with_delimiter(object.member_count)
   end
+
+  private
+
+  def membership
+    return unless request.current_user
+
+    @membership ||= request.current_user.membership_for_object(object)
+  end
 end

spec/serializers/group_child_entity_spec.rb

@@ -34,7 +34,7 @@ describe GroupChildEntity do
   end
 
   describe 'for a project' do
-    set(:object) do
+    let(:object) do
       create(:project, :with_avatar,
              description: 'Awesomeness')
     end
@@ -55,7 +55,7 @@ describe GroupChildEntity do
   end
 
   describe 'for a group', :nested_groups do
-    set(:object) do
+    let(:object) do
       create(:group, :nested, :with_avatar,
              description: 'Awesomeness')
     end