Set default values for group

app/models/course/group.rb

@@ -1,8 +1,18 @@
 class Course::Group < ActiveRecord::Base
   stampable
 
+  before_validation :set_defaults, if: :new_record?
+
   belongs_to :course, inverse_of: :groups
   has_many :group_users, inverse_of: :course_group, dependent: :destroy,
                          class_name: Course::GroupUser.name, foreign_key: :course_group_id
   has_many :users, through: :group_users
+
+  private
+
+  # Set default values
+  def set_defaults
+    return unless creator && creator.courses.include?(course) && group_users.empty?
+    group_users.build(user: creator, role: :manager, creator: creator, updater: updater)
+  end
 end