title: How to validate unique constraint on django rest framework?
body: <p>How can i validate an unique constraint with a key that is not on the request payload?</p>
  
  <p>The key that i need to validate are <code>user_id</code> and <code>sku</code> but the request does not contain the <code>user_id</code> key.</p>
  
  <p>Example of payload:</p>
  
  <pre><code>{'sku': '123', data: []}
  </code></pre>
  
  <p>The serializers:</p>
  
  <pre><code>class ProductConfiguration(serializers.Serializer):
  
      min_quantity = serializers.IntegerField(required=True)
      price = serializers.DecimalField(
          required=True,
          decimal_places=2,
          max_digits=10,
          coerce_to_string=False
      )
  
  
  class ProductSerializer(serializers.ModelSerializer):
  
      sku = serializers.CharField(required=True)
      data = ProductConfiguration(many=True, required=True)
  
      class Meta:
          model = WholeSale
  
          # the "id" and "user_id" columns should not be included on the response
          exclude = ['id', 'user']
  </code></pre>
  
  <p>I need to validate that the <code>user</code> and <code>sku</code> key already exist.</p>
  
  <p>By default if the two keys <code>user_id</code> and <code>sku</code> were on the payload <code>drf</code> could take care of <code>Unique</code> error, how can i validate this two keys if one of them are not on the payload?</p>