Removing object from association collection.rb

class Product
  include Mongoid::Document
  referenced_in :group
end

class Group
  include Mongoid::Document
  references_many :products, :stored_as => :array, :inverse_of => :group
end

group = Group.first
product = group.products.last

group.products.size # => 5
group.products.delete(product)
group.save

group.products.size # => 5