ruby-on-rails – Ruby on Rails：为什么选择框不显示当前对象值

<%= form_for(:product,:url => {:action => 'update',:id => @product.id}) do |f| %>
  <%= render(:partial => "form",:locals => {:f => f}) %>
  <%= submit_tag("Update Product") %>
<% end %>

来自views / products / _form.html.erb：

<%= select_with_new_option(f,:shop,:name,:id) %>

从helpers / products_helper.rb：

def select_options_with_create_new(objects,text_field,value_field,options={})
  object = objects.to_s.singularize
  all_objects = object.capitalize.constantize.all
  all_objects = all_objects.sort_by(&options[:order_by]) if options.has_key?(:order_by)
  select_options = all_objects.map{ |obj| [obj.send(text_field),obj.send(value_field)] }
  select_options << [wrap_create_new_option("create new #{object}".titleize),"new_#{object}"]
  options_for_select(select_options)
end

def wrap_create_new_option(str)
  ">> #{str} <<"
end

# By default,sorts by 'text_field'.
def select_with_new_option(f,object,value_field)
  f.select(:"#{object}_id",select_options_with_create_new(object.pluralize,:order_by => text_field))
end

我希望默认情况下选择框为@ product.shop_id,但这不是真的(第一个选项始终是默认值).

我错过了什么？