test.rb

class Parent
  def render(args={})
    expand = args[:expand] || {}
    obj = { }
    # how would we access child.fields here?
    Parent.fields.each do |name, options|
      prop = self.send(name)
      obj[name] = if options[:model]
                    if (inner = expand[name])
                      prop.render(expand: inner)
                    else
                      {
                        id: prop.id
                      }
                    end
                  elsif options[:format] == :date
                    prop.utc.to_s
                  else
                    prop
                  end
    end
    obj
  end
  def self.show
    puts @@fields.inspect
  end
  protected
  def self.field(name, options={})
    attr_accessor name
    @fields ||= {}
    @fields[name] = options
  end
end

class A < Parent
  field :hi, foo: 2
end

class B < Parent
  field :boo, derp: 3
end

b = B.new
b.boo = "heyo"
# this line breaks because of the issue I am trying to solve
# test2.rb:6:in `block in render': undefined method `hi' for #<B:0x00000000c23fd0 @boo="heyo"> (NoMethodError)
puts b.render

puts "B fields"
puts B.show

puts "A fields"
puts A.show

puts "Parent fields"
puts Parent.show

# desired output:
# B fields
# {:boo => {:derp => 3}}
# A fields
# {:hi => {:foo => 2}}
# Parent fields
# {}