title: cannot assign must be a user instance django
body: <p>I have created a model for form submission which is working fine, I want to create a new model for the submission of documents based on the respective form using the Form id as <strong>Foreign Key</strong> in the documents model</p>
  
  <p>My documents model is as follow:</p>
  
  <pre><code>class PHDDocument(models.Model):
      user = models.ForeignKey(HigherStudiesForms, on_delete=models.PROTECT, null=True, blank=True)
      admission_letter = models.FileField(blank=False, null=False)
      award_letter = models.FileField(blank=False, null=False)
      attested_degree = models.FileField(blank=False, null=False)
      cv = models.FileField(blank=False, null=False)
      travel_agent = models.FileField(blank=False, null=False)
      surety_bond = models.FileField(blank=False, null=False)
      attested_surety_bond = models.FileField(blank=False, null=False)
      guarantee_cnic = models.FileField(blank=False, null=False)
      study_leave = models.FileField(blank=False, null=False)
      objection_certificate = models.FileField(blank=False, null=False)
      hod_form = models.FileField(blank=False, null=False)
      rector_form = models.FileField(blank=False, null=False)
  
      def __str__(self):
          return str(self.admission_letter)
  </code></pre>
  
  <p>view method respect to the following model is as below:</p>
  
  <pre><code>class PHDFileView(viewsets.ModelViewSet):
      permission_classes = (IsAuthenticated,)
      lookup_field = 'pk'
      lookup_url_kwarg = 'form_id'
      serializer_class = PHDDocumentSerializer
  
      def get_queryset(self):
          queryset = PHDDocument.objects.all()
          return queryset
  
      def perform_create(self, serializer):
          serializer.save(user=self.request.user)
  </code></pre>
  
  <p>The serializer method is as follows:</p>
  
  <pre><code>class PHDDocumentSerializer(serializers.ModelSerializer):
  class Meta:
      model = PHDDocument
      fields = ('id', 'admission_letter', 'award_letter', 'attested_degree', 'cv', 'travel_agent', 'surety_bond',
                'attested_surety_bond', 'guarantee_cnic', 'study_leave', 'objection_certificate', 'hod_form',
                'rector_form')
  </code></pre>
  
  <p>when I am submitting the documents after the form submission through Angular Django is giving the following error <strong>"ValueError: Cannot assign "User: ahssu": "PHDDocument.user" must be a "HigherStudiesForms" instance."</strong></p>