Arrays attributes don't update

What is the mistake in this code for not be able to update an array within a document?
Model

class Foo

  include Mongoid::Document

  include Mongoid::Timestamps::Created

  field :myarray, type: Array

end

Controller

def add_item

        @foo = Foo.find_by(uuid: params[:uuid])

        unless @foo.nil?

            unless @foo.has_attribute? :myarray

                @foo[:myarray] = Array.new

            end

            @foo[:myarray] << params[:item]

            @foo.save

        end

end

I am using Rails 4 with MongoId 4 and if I do {{ p @foo }} before {{ @foo.save }} I can see @foo correctly changed but for any reason the update is not persisted.

Otherwise if I change {{ @foo[:myarray] << params[:item] }} by {{ @foo[:myarray] += [ params[:item] ] }} then all works fine.

I'm missing something in the first way?