title: Django admin:custom search
body: <p>i have tow model:  </p>
  
  <pre><code>class Site_Posts(models.Model):
      title = models.CharField(max_length=300,default='')
      ...
      def __unicode__(self):
          return self.title
  
  class Comment(models.Model):
      post_title = models.ForeignKey(Site_Posts,null=True)
      post_id = models.IntegerField(null=True,default=None)
      ...
  
  class Comment_Admin_Form(admin.ModelAdmin):
      model = Comment
      search_fields = ('content','post_title__title',)
      ...
  </code></pre>
  
  <p>i want to show all comment that <code>post_title</code> field's is equal to text of admin search box widget,but now displayed just one(Whereas there are exist many item)</p>