rep.rb

require 'representable/json'

Representable.deprecations = false

class UserRepr < Representable::Decorator
  include Representable::JSON

  property :name
end

class UserFirstNameRepr < UserRepr
  property :first_name, getter: ->(_){ name[/\S+/] }
end

class User < Struct.new(:name)
end

user = User.new('Iron Fist')
puts UserRepr.new(user).to_json          # {"name":"Iron Fist"}
puts UserFirstNameRepr.new(user).to_json # {"name":"Iron Fist","first_name":"Iron"}