title: Run Serializer's validation only when creating a new post
body: <p>I have 2 views - one of them uses <code>create</code> and another one uses <code>RetrieveUpdateDestroyAPIView</code>. Both use same Serializer class. I need title validation only when creating a new post, not when updating a post.<br>
  I have title validation error when I'm updating some post with "PUT" request.<br>
  How can I fix this?</p>
  
  <pre><code>class StoreApiView(mixins.CreateModelMixin, generics.ListAPIView):
      lookup_field = 'pk'
      serializer_class = ProductSerializer
  
      def get_queryset(self):
          qs = Product.objects.all()
          query = self.request.GET.get('q')
          if query is not None:
              qs = qs.filter(
                  Q(title__icontains=query) |
                  Q(description__icontains=query)
              ).distinct()
          return qs
  
      def perform_create(self, serializer):
          serializer.save(author=self.request.user)
  
      def post(self, request, *args, **kwargs):
          return self.create(request, *args, **kwargs)
  
  
  class StoreRucView(generics.RetrieveUpdateDestroyAPIView):
      lookup_field = 'pk'
      serializer_class = ProductSerializer
      permission_classes = [IsOwnerOrReadOnly]
  
  
      def get_queryset(self):
          return Product.objects.all()
  </code></pre>
  
  <p>This is the serializer class full code: </p>
  
  <pre><code>class ProductSerializer(ModelSerializer):
      product_ratings = ProductRatingSerializer(many=True,read_only=True)
      product_badges_set = ProductBadgesSerializer(many=True,read_only=True)
  
      author = serializers.SerializerMethodField()
      category = serializers.SerializerMethodField()
  
      def get_author(self, obj):
          return obj.author.first_name+' '+obj.author.last_name
  
      def get_category(self, obj):
          return obj.category.title
  
      class Meta:
          model = Product
          fields = [
              'product_id',
              'author',
              'category',
              'title',
              'description',
              'price',
              'level',
              'video_length',
              'created_date',
              'updated_date',
              'product_ratings',
              'product_badges_set',
          ]
          read_only_fields = ['product_id', 'created_date', 'updated_date', 'author']
  
      def validate_title(self, value):
  
          qs = Product.objects.filter(title__iexact=value)
          if self.instance:
              qs.exclude(pk=self.instance.pk)
          if qs.exists():
              raise serializers.ValidationError("this title is already used")
          return value
  </code></pre>