ruby-on-rails-3 – Rails：连接表中两个属性的唯一性导致500错

class Subject < ActiveRecord::Base
  # Self Associations
  has_many :subcategories,:class_name => "Subject"
  belongs_to :category,:class_name => "Subject",:foreign_key => "parent_id"

  # Associations
  has_many :knowledges
  has_many :professors,:through => :knowledges
end


class Professor < ActiveRecord::Base
  # Associations
  has_many :knowledges
  has_many :subjects,:through => :knowledges
  ...
end

class Knowledge < ActiveRecord::Base
  # Associations
  belongs_to :professor
  belongs_to :subject
  has_one :level

  attr_accessible :subject_id,:professor_id

  validates :subject_id,:uniqueness => { :scope => :professor_id }
end

我希望有一个表格可以让教授在他的帐户中添加一个主题,我决定有一个知识形式(因为我希望能够插入一个级别).

它看起来像这样：

<%= simple_form_for @knowledge,:url => professor_knowledges_path,:html => { :class => 'form-horizontal' } do |f| %>
    <div class="control-group select optional">
      <%= label_tag "Subject Type",nil,:class => "select optional control-label"%>
      <div class="controls">
    <%= select_tag "Parent Subject",options_from_collection_for_select(@parent_subjects,"id","name"),:id => "knowledge_parent_subject" %>
      </div>
    </div>
    <%= f.input :subject_id,:collection => @subjects,:label => "Subject" %>
    <%= f.input :level %>
  <%= f.button :submit,t('add_form'),:class => 'btn-primary' %>
<% end %>

在Knowledges控制器的创建动作中,我有：

def create
    @knowledge = Knowledge.create(:professor_id => current_professor.id,:subject_id => params[:knowledge][:subject_id]) 
  end

我希望/期望得到一个ActiveRecord说这个知识无法插入,因为存在唯一性违规,但是nops,我只看到日志中的500和回滚,但似乎执行继续.所以我的问题是：我做错了什么,或者我如何改进这种建模情况？我认为表单需要与连接模型相关,因为我想在其上设置该模型的字段……但也许我错了,我可以用简单/清洁的方式做.

编辑：

正如其中一条评论中所述,这里是回滚后提交表单和500错误的日志：

Started POST "/professors/1/knowledges" for 127.0.0.1 at 2012-07-01 00:45:39 -0700
Processing by KnowledgesController#create as HTML
  Parameters: {"utf8"=>"?","authenticity_token"=>"4JVyxWnIh37kyBwLwLGTHk/znsI1c5wrJvaWjKKT5tM=","Parent Subject"=>"1","knowledge"=>{"subject_id"=>"1"},"commit"=>"A?adir","professor_id"=>"1"}
  Professor Load (0.4ms)  SELECT `professors`.* FROM `professors` WHERE `professors`.`id` = 1 LIMIT 1
Completed 500 Internal Server Error in 4ms

我在create动作中添加了一些条件,如下所示：

def create
    @knowledge = Knowledge.new(:professor_id => current_professor.id,:subject_id => params[:knowledge][:subject_id]) 
    if @knowledge.save
      flash[:notice] = "Success..."
      redirect_to professor_path(current_professor)
    else
      render :action => 'new'
    end
  end

这实际上在500之后显示以下内容：

Completed 500 Internal Server Error in 6ms

ActiveRecord::RecordInvalid (Validation failed: Subject has already been taken):

我想知道为什么会引发异常,而不是仅仅将错误添加到对象中,让我管理这种情况.不是以下行应该做什么？

validates :subject_id,:uniqueness => { :scope => :professor_id }