title: How to post relationship field in DRF
body: <p>Here is models</p>
  
  <pre><code>class Event(models.Model):
      calendar = models.ForeignKey(Calendar)
      title = models.CharField()
  
  class Calendar(models.Model):
      title = models.CharField()
  </code></pre>
  
  <p>Here is serializers</p>
  
  <pre><code>class EventSerializer(models.Model):
      calendarId = serializers.CharField(source='calendar.id')
      class Meta:
          model = Event
          fields = ('id', 'title', 'calendarId')
  
  class CalendarSerializer(models.Model):
      class Meta:
          model = Calendar
          fields = ('id', 'title')
  </code></pre>
  
  <p>Then I try to post below json to create an event under calendar <code>3b60a22c-d46e-46ca-b978-ec81a8b47fcb</code></p>
  
  <pre><code>{
      title:"abc",
      calendarId:"3b60a22c-d46e-46ca-b978-ec81a8b47fcb"
  }
  </code></pre>
  
  <p>It failed with below error</p>
  
  <blockquote>
    <p>ValueError: Cannot assign "{u'id':
    u'3b60a22c-d46e-46ca-b978-ec81a8b47fcb'}": "Event.calendar" must
    be a "Calendar" instance.</p>
  </blockquote>
  
  <p>How to fix this issue?</p>