What if, we've more pictorial relationship syntax than syntactical sugar ?

with_default_mongoid_methods.rb

class Message
  include Mongoid::Document
  include Mongoid::Timestamps

  field :subject, type: String
  field :content, type: String

  belongs_to              :replied_to, class_name: 'Message', inverse_of: :replies, touch: true
  belongs_to              :sender, class_name: 'User', inverse_of: :sent_messages
  has_many                :replies, class_name: 'Message', inverse_of: :replied_to
  has_and_belongs_to_many :recipients, class_name: 'User', inverse_of: :received_messages
end

with_friendly_relationship.rb

class Message
  include Mongoid::Document
  include Mongoid::Timestamps

  field :subject, type: String
  field :content, type: String

  relation 'Message => self',       as: :replied_to, touch: true
  relation 'User => self',          as: :sent_messages  
  relation 'self => Message',       as: :replies, inverse_of: :replied_to
  relation 'self => User <= self',  as: :recipients, inverse_of: :received_message 
end