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.其他工作正常但如果条件为0则告诉我没有方法＆＃39; id＆＃39;。

- 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>