title: Issues with saving Nested ManyToMany relationship with ModelSerializer
body: <p>I have the following model:</p>
  <pre class="lang-py prettyprint-override"><code>class Collection(models.Model):
      ...
      relationships = models.ManyToManyField('relationships.Relationships', related_name='collections', blank=True)
      ...
  </code></pre>
  <p>I also have the following serializer:</p>
  <pre><code>class CollectionSerializer(serializers.ModelSerializer):
      relationships = RelationshipSerializer(many=True)
  
      class Meta:
          model = Collection
          fields = [
              ...
              'relationships',
              ...
          ]
          read_only_fields = [
              ...
          ]
  
      def create(self, validated_data):
          relationships = validated_data.pop('relationships')
  
          # Then we need to create an instance of the Collection object from the validated data:
          collection = Collection.objects.create(**validated_data)
  
          for relationship in relationships:
              relationship = Relationships.objects.get(uuid=relationship['uuid'])
              collection.relationships.add(relationship.id)
  
          return collection
  </code></pre>
  <p>A sample body I am posting to the endpoint where this serializer is called is:</p>
  <pre><code>{
      &quot;relationships&quot;: [
          {
              &quot;name&quot;: [
                  &quot;Relationship with this Name already exists.&quot;
              ]
          },
          {
              &quot;name&quot;: [
                  &quot;Relationship with this Name already exists.&quot;
              ]
          }
      ]
  }
  </code></pre>
  <p>Even when I set the relationships field to read only it throws this error?</p>
  <p>What have I done wrong?</p>