Return number of guests per group

app/controllers/groups_controller.rb

@@ -1,5 +1,6 @@
 class GroupsController < ApplicationController
   def index
-    render jsonapi: Group.where(parent_id: nil), include: [children: [children: [:children]]]
+    roots = Group.where(parent_id: nil)
+    render jsonapi: roots, include: [children: [children: [:children]]]
   end
 end

app/serializers/serializable_group.rb

@@ -4,4 +4,8 @@ class SerializableGroup < JSONAPI::Serializable::Resource
   attributes :name, :icon
 
   has_many :children
+
+  attribute :guest_count do
+    @object.guests.count
+  end
 end