title: Django rest framework validation when updating nested serializer (many=True)
body: <p>I have the following models:</p>
  
  <pre><code>class Child(models.Model):
      attr1 = models.CharField(max_length=20)
      attr2 = models.CharField(max_length=30)
      attr3 = models.BigIntegerField()
  
      class Meta:
          unique_together = ("attr1", "attr2", "attr3")
  
  class Parent(models.Model):
      children = models.ManyToManyField(Child, related_name="parents")
  </code></pre>
  
  <p>Then the following serializers:</p>
  
  <pre><code>class ChildSerializer(serializers.ModelSerializer):
      class Meta:
          model = Child
  
  class ParentSerializer(serializers.ModelSerializer):
      children = ChildSerializer(many=True)
      class Meta:
          model = Parent
  </code></pre>
  
  <p>When updating the parent instance with the nested (many=True) children serializer, I get a validation error:</p>
  
  <pre><code>The fields attr1, attr2, attr3 must make a unique set.
  </code></pre>
  
  <p>However, shouldn't the serializer skip validation for the children since they're already retrieved instances from the database?</p>