foo.rb

class ActiveModel::Serializer
  def initialize(object, options = {})
    @object = object
    @include_directive = options[:include]
  end
  
  def as_json
    hash = serialize_attributes(@object)
    self.relationships.each_pair do |name, relationship|
      related_record = object.send(name)
      hash[:relationships][name] = relationship.serializer.new(related_record, @include_directive[name])
    end
    hash
  end
end