title: Creating and saving foreign key objects using a SlugRelatedField
body: <p>I've just started with Django REST framework and I'm having trouble with saving foreign keys. I have a <code>Merchant</code> model and a <code>Phone</code> model. The <code>Phone</code> has a foreign key to <code>Merchant</code>. When making a <code>POST</code> request to <code>Merchant</code>, I want to create <code>Phone</code> objects for the numbers provided in the request. But when I supply the phone numbers, it gives me the following error</p>
  
  <blockquote>
    <p>Object with phone=0123456789 does not exist.</p>
  </blockquote>
  
  <p>I just want it to create the <code>Phone</code> object itself. Here are the models that I am using:</p>
  
  <pre><code>class Merchant(models.Model):
      merchant_id       = models.CharField(max_length=255)
      name              = models.CharField(max_length=255)
      is_active         = models.BooleanField(default=True)
  
      class Meta:
          managed = True
          db_table = 'merchant'
  
      # Managers
      objects = models.Manager()
      active = managers.ActiveManager()
  
  class Phone(models.Model):
      phone      = models.CharField(max_length=255)
      merchant   = models.ForeignKey('merchant.Merchant',
                                      related_name='phones',
                                      blank=True,
                                      null=True)
  
      class Meta:
          managed = True
          db_table = 'phone'
  </code></pre>
  
  <p>And here is the view and serializer that I am using them with</p>
  
  <pre><code>class MerchantSerializer(serializers.ModelSerializer):
      phones = serializers.SlugRelatedField(
          many=True,
          slug_field='phone',
          queryset=primitives.Phone.objects.all())
  
      class Meta:
          model = Merchant
          fields = (
              'merchant_id',
              'name',
              'is_active',
              'phones',
          )
  
  class MerchantViewSet(viewsets.ModelViewSet):
      queryset = Merchant.active.all()
      serializer_class = MerchantSerializer
  </code></pre>
  
  <p>Here's what my request body looks like:</p>
  
  <pre><code>{
      "merchant_id": "emp011",
      "name": "Abhinav",
      "is_active": true,
      "phones": [
          "0123456789",
          "9876543210"
      ]
  }
  </code></pre>
  
  <p>Here's the response:</p>
  
  <p>400 Bad Request</p>
  
  <pre><code>{"phones":["Object with phone=0123456789 does not exist."]}
  </code></pre>