Mongoid Observer using ActiveModel.

mongoid.rb

require "#{RAILS_ROOT}/lib/mongoid/lib/observing.rb"

# Initializer for Observers

User.observers = :user_observer
User.instantiate_observers

# The preferred syntax, similar to AR's setup
# Mongoid.configure do |config|
#   config.observers = :user_observer
# end

observing.rb

module Mongoid
  module Observing
    CALLBACKS = [
      :before_create, :before_destroy, :before_save, :before_update,
      :before_validation, :after_create, :after_destroy, :after_save,
      :after_update, :after_validation
    ]

    def self.included(base)
      base.module_eval { include ActiveModel::Observing }

      CALLBACKS.each do |callback|
        callback_method = :"notify_observers_#{callback}"

        base.module_eval <<-RUBY, __FILE__, __LINE__+1
          def #{callback_method}(&block)
            notify_observers(#{callback.inspect}, &block)
          end
          private #{callback_method.inspect}
        RUBY
      end
    end
  end
end

user.rb

require 'carrierwave/orm/mongoid'  

class User
  include Mongoid::Document
  include Mongoid::Timestamps
  include Mongoid::Observing
  
  field :name
  field :username
end

user_observer.rb

class UserObserver < ActiveModel::Observer  
  def after_validation(user)
    puts "User Observer After Validation"
  end
end