ActiveModel::Observer and ActiveRecord...

If you want to use ActiveModel observers outside of ActiveRecord, you will need to include the ActiveModel observer model as such:

class Post
  include MongoMapper::Document
  include ActiveModel::Observing

  def create(*)
    notify_observers(:before_create, self)
    value = super
    notify_observers(after_create, self)
    value
  end
end

If you wish, you can create a module that you can mix in to any MongoMapper model with observing powers:

module MongoMapper
  module Observing
    # This will make sure that ActiveModel::Observing is included onto any model
    # that MongoMapper::Observing is included onto
    extend ActiveSupport::Concern
    include ActiveModel::Observing

    def create(*)
      notify_observers(:before_create, self)
      value = super
      notify_observers(after_create, self)
      value
    end

  end
end

class Post
  include MongoMapper::Document
  include MongoMapper::Observer
end

That said, the MongoMapper folks should probably provide something like this out of the box.

[bulk edit]