Validates existence of in MongoMapper

validates_existence_of.rb

module ValidatesExistenceOfPlugin
  def self.included(model)
    model.plugin ValidatesExistenceOfPlugin
  end
  
  module ClassMethods
    def validates_existence_of(*args)
      add_validations(args, ValidatesExistenceOfPlugin::ValidatesExistenceOf)
    end
  end
  
  class ValidatesExistenceOf < Validatable::ValidationBase
    def valid?(instance)
      instance.send(attribute).present?
    end

    def message(instance)
      super || "does not exist"
    end
  end
end

MongoMapper::Document.append_inclusions(ValidatesExistenceOfPlugin)
MongoMapper::EmbeddedDocument.append_inclusions(ValidatesExistenceOfPlugin)

Example usage in combination with belongs_to's:

belongs_to :user
belongs_to :article
validates_existence_of :article
validates_existence_of :user

This is stupid actually. Just use validates_presence_of. Sometimes one over thinks...