title: Django - get object based on ForeignKey field
body: <p>I need to get a single model object based on a set of fields one of which is a <code>ForeignKey</code>. So if I have the following models:</p>
  
  <pre><code>class Composer(models.Model):
      name = models.CharField()
      dates =  models.CharField()
      nationality = models.CharField()
      period = models.CharField()
  
  class Symphony(models.Model):
      composer = models.ForeignKey('Composer', related_name="symphonies", null=True)
      date = models.CharField()
      key = models.CharField()
      number = models.IntegerField()
      num_movements = models.IntegerField()
  </code></pre>
  
  <p>how can I then retrieve a specific <code>Symphony</code> object based on its <code>composer</code> and <code>number</code> fields? I originally had composer as a simple <code>models.CharField()</code>, so I could just do:</p>
  
  <pre><code>Symphony.objects.get(composer='Bruckner', number=7)
  </code></pre>
  
  <p>So how do I do the equivalent using a <code>ForeignKey</code>?</p>