title: How to do in operation in django
body: <pre><code>class ModelA(models.Model):
      f = models.CharField(max_length=50)
  
  class ModelB(models.Model):
      a = ForeignKey(ModelA)
      b = models.CharField(max_length=50)
  </code></pre>
  
  <p>I want to get f in model A and I know b . How can I do this by using django orm ?</p>