title: Django-rest-framework serializer fails to save model instance
body: <p>I am using DRF. 
  My models.py:</p>
  
  <pre><code>class Farm(models.Model):
      farmer = models.ForeignKey(User, verbose_name="Farmer of this farm")
      name = models.CharField(max_length=15, blank=False, verbose_name="Farm name", help_text="Please give a single word name for your farm.")
      address = models.TextField(help_text="Please provide the address without the postal code", blank=False,)
      postal_code = models.CharField(max_length=5, blank=False, default='12043')
      size = models.DecimalField(max_digits=9, decimal_places=6, blank=False, )
  
      #To add user's full name in the admin interface for better readability
      def get_farmer_full_name(self):
          return self.farmer.get_full_name()
  
      #Works like a verbose_name but for a method 
      get_farmer_full_name.short_description = 'Farmers full name'
  </code></pre>
  
  <p>My serializers.py:</p>
  
  <pre><code>class FarmSerializer(serializers.ModelSerializer):
  
      class Meta:
              model = Farm
              fields = ('farmer', 'name', 'address', 'postal_code', 'size')
  
      def create(self, validated_data):
          return Farm.objects.create(**validated_data)
  
      def update(self, instance, validated_data):
          instance.name = validated_data.get('name', instance.name)
          instance.address = validated_data.get('address', instance.address)
          instance.postal_code = validated_data.get('postal_code', instance.postal_code)
          instance.size = validated_data.get('size', instance.size)
          instance.save()
          return instance 
  </code></pre>
  
  <p>My views.py:</p>
  
  <pre><code>@api_view(('POST',))
  def addFarm(request):
      data = JSONParser().parse(request)
      farmer = Farm(farmer = request.user)
      serializer = FarmSerializer(farmer, data=data)
      if serializer.is_valid():
          serializer.save()
          return Response(serializer.data, status=status.HTTP_201_CREATED)
      return Response(serializer.errors, status=status.HTTP_400_BAD_REQUEST)
  </code></pre>
  
  <p>When I hit <code>url(r'^farms/save/$', views.addFarm),</code> this url, I want to save a new Farm instance with the parameters provided in request. I am using <em>Postman</em> to send the request. </p>
  
  <p>However I get <code>{"farmer":["This field is required."],"name":["This field is required."],"address":["This field is required."],"size":["This field is required."]}</code>  error as response. </p>
  
  <p>My request looks like following:</p>
  
  <p><a href="https://i.stack.imgur.com/KUCNu.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/KUCNu.png" alt="enter image description here"></a></p>
  
  <p><strong><em>Note:</em></strong> The first field 'farmer' I can not send as a parameter but I guess I have to extract it with <code>request.user.id</code> and pass it exclusively, however I am not sure.</p>
  
  <p>What am I doing wrong? How can this be fixed?</p>