Dynamically extend Virtus instance attributes - ruby-on-rails

Question

Let's say we have a Virtus model User
class User
include Virtus.model
attribute :name, String, default: 'John', lazy: true
end
Then we create an instance of this model and extend from Virtus.model to add another attribute on the fly:
user = User.new
user.extend(Virtus.model)
user.attribute(:active, Virtus::Attribute::Boolean, default: true, lazy: true)
Current output:
user.active? # => true
user.name # => 'John'
But when I try to get either attributes or convert the object to JSON via as_json(or to_json) or Hash via to_h I get only post-extended attribute active:
user.to_h # => { active: true }
What is causing the problem and how can I get to convert the object without loosing the data?
P.S.
I have found a github issue, but it seems that it was not fixed after all (the approach recommended there doesn't work stably as well).

Answer 1

Building on Adrian's finding, here is a way to modify Virtus to allow what you want. All specs pass with this modification.
Essentially, Virtus already has the concept of a parent AttributeSet, but it's only when including Virtus.model in a class.
We can extend it to consider instances as well, and even allow multiple extend(Virtus.model) in the same object (although that sounds sub-optimal):
require 'virtus'
module Virtus
class AttributeSet
def self.create(descendant)
if descendant.respond_to?(:superclass) && descendant.superclass.respond_to?(:attribute_set)
parent = descendant.superclass.public_send(:attribute_set)
elsif !descendant.is_a?(Module)
if descendant.respond_to?(:attribute_set, true) && descendant.send(:attribute_set)
parent = descendant.send(:attribute_set)
elsif descendant.class.respond_to?(:attribute_set)
parent = descendant.class.attribute_set
end
end
descendant.instance_variable_set('#attribute_set', AttributeSet.new(parent))
end
end
end
class User
include Virtus.model
attribute :name, String, default: 'John', lazy: true
end
user = User.new
user.extend(Virtus.model)
user.attribute(:active, Virtus::Attribute::Boolean, default: true, lazy: true)
p user.to_h # => {:name=>"John", :active=>true}
user.extend(Virtus.model) # useless, but to show it works too
user.attribute(:foo, Virtus::Attribute::Boolean, default: false, lazy: true)
p user.to_h # => {:name=>"John", :active=>true, :foo=>false}
Maybe this is worth making a PR to Virtus, what do you think?

Answer 2

I haven't investigated it further, but it seems that every time you include or extend Virtus.model, it initializes a new AttributeSet and set it to #attribute_set instance variable of your User class (source). What the to_h or attributes do is they call the get method of the new attribute_set instance (source). Therefore, you can only get attributes after the last inclusion or the extension of Virtus.model.
class User
include Virtus.model
attribute :name, String, default: 'John', lazy: true
end
user = User.new
user.instance_variables
#=> []
user.send(:attribute_set).object_id
#=> 70268060523540
user.extend(Virtus.model)
user.attribute(:active, Virtus::Attribute::Boolean, default: true, lazy: true)
user.instance_variables
#=> [:#attribute_set, :#active, :#name]
user.send(:attribute_set).object_id
#=> 70268061308160
As you can see, the object_id of attribute_set instance before and after the extension is different which means that the former and the latter attribute_set are two different objects.
A hack I can suggest for now is this:
(user.instance_variables - [:#attribute_set]).each_with_object({}) do |sym, hash|
hash[sym.to_s[1..-1].to_sym] = user.instance_variable_get(sym)
end