Question

I’m using Virtus (basically the Property API from DataMapper) to build around a remote API, however I don’t think Virtus is the problem here, I think it’s my lack of understanding of what Ruby is doing.

I want to allow attributes that coerce to a given type, by using the Syntax:

Node[AnotherClass]

Which simply generates on-the-fly a subclass of Node and returns that new class. That part is working. But for some reason, it has an undesirable side-effect. All other objects that descend from Virtus::Attribute::Object are actually also Node subclasses themselves. I can’t explain it, but I think it must be an expected ruby behaviour related to the inheritance model. Can anybody point me in the right direction?

(Note the following code runs without modification if you gem install virtus).

require "virtus"

class JsonModel
  include Virtus
end

class Node < Virtus::Attribute::Object
  attr_reader :type

  class << self
    def [](type)
      raise ArgumentError, "Child nodes may only be other JsonModel classes" unless type <= JsonModel

      @generated_class_map       ||= {}
      @generated_class_map[type] ||= Class.new(self) do
        default lambda { |m, a| type.new }

        define_method :type do
          type
        end

        define_method :coerce do |value|
          value.kind_of?(Hash) ? type.new(value) : value
        end
      end
    end
  end
end

class ChildModel < JsonModel
end

class ParentModel < JsonModel
  attribute :child,  Node[ChildModel]
  attribute :string, String
end

# This should be String, but it's a descendant of Node??
puts ParentModel.attributes[:string].class.ancestors.inspect

Answer 1

I’ve reduced your code down to the following, and it still behaves the same.

The presence of Class.new(Node) is what causes the :string attribute to have Node in its ancestry.

require "virtus"

class JsonModel
  include Virtus
end

class Node < Virtus::Attribute::Object
end

# this does it...
Class.new(Node)

class ParentModel < JsonModel
  attribute :string, String
end

# This should be String, but it's a descendant of Node??
puts ParentModel.attributes[:string].class.ancestors.inspect

I’m not familiar with Virtus, but I’m guessing it’s to do with the way Virtus implements attribute types.