title: How to override the queryset giving the filters in list_filter?
body: <p>Given the following models</p>
  
  <pre><code>class AnotherModel(models.Model):
      n = models.IntegerField()
  
  class MyModel(models.Model):
      somefield = models.ForeignKey(AnotherModel)
  </code></pre>
  
  <p>and admin</p>
  
  <pre><code>class MyModelAdmin(admin.ModelAdmin):        
      list_filter = ('somefield',)
  </code></pre>
  
  <p>how can I filter the instances of <code>AnotherModel</code> to show only those with a given <code>n</code> value in my admin filter?</p>
  
  <p>I need something like:</p>
  
  <p>Filter</p>
  
  <p>By somefield</p>
  
  <p>all</p>
  
  <p>[list of AnotherModel instances with given n]</p>