请教Rails中的create(attributes={})的问题

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下面是添加student的代码

def add
   @student = Student.new(params[:student])
   @klass = Klass.find(:first, :conditions => [ "klass_num = ?" , params[:klass][:klass_num] ]) 
   if @klass.students.create(@student.attributes) 
     flash[:notice] = "Student was successfully created"
     redirect_to :action => :list
   else
     flash[:notice] = "Failed to create student!"
     redirect_to :action => :add_student
   end
end

@klass.students.create(@student.attributes) 

这句本来不应该是把@student关联到@klass对象,并且保存新建的对象@student么？
但是结果并没有保存，弄了半天也没找出什么原因。
我把

if @klass.students.create(@student.attributes) 

我改成：

@klass.students.build(@student.attributes)
if @student.save

保存是成功了，但是没有建立klass与student之间的关联，不知道到底哪儿有问题，希望大家指点一下
log中传递的参数是正确的：
Parameters: {"klass"=>{"klass_num"=>"210309"}, "commit"=>"添加", "action"=>"add", "controller"=>"student", "student"=>{"name"=>"fuliang", "password_confirmation"=>"123456", "student_num"=>"21030922", "password"=>"123456"}}