ruby-on-rails - Rails collection_select NoMethodError

Question

在我看来，我有一个 collection_select。我希望它提示用户选择会话变量是否设置为 0。 else 工作正常，但如果条件为 0，那么它告诉我没有方法“id”。

- if session[:ptype_id] == 0
  = collection_select :ptype, :id, Ptype.all.order(:part_type), :id, :part_type, {:prompt => "Select Type"}, :onchange => "ptype_set(value)"
- else
  = collection_select :ptype, :id, Ptype.all.order(:part_type), :id, :part_type, {:selected => session[:ptype_id]}, :onchange => "ptype_set(value)"

错误...

undefined method `id' for #<Ptype::ActiveRecord_Relation:0x007f9bcf138428>