如何在表单中返回适当的嵌套属性值(编辑操作)?

I have following structure:

我有如下结构:

class Project < ActiveRecord::Base
  has_many :costs, :dependent => :destroy
  accepts_nested_attributes_for :costs, :allow_destroy => true
end

class Cost < ActiveRecord::Base
  belongs_to :project
end
class P