title: Django - Column 'user_id' cannot be null
body: <p>An error occurs when I try to save the data.</p>
  
  <p>Column 'user_id' cannot be null.</p>
  
  <p>Below are the settings I'm using for the Model</p>
  
  <pre><code>class XML(models.Model):
      nome_destinatario = models.CharField(max_length=255)
      cnpj_destinatario = models.CharField(max_length=15)
      user = models.ForeignKey(User)
  
      def __str__(self):
          return self.nome_destinatario
  </code></pre>
  
  <p>Serializer</p>
  
  <pre><code>class XMLCreateSerializer(serializers.ModelSerializer):
      class Meta:
          model = XML
          fields = ('nome_destinatario', 'cnpj_destinatario', 'user_id')
  </code></pre>
  
  <p>View</p>
  
  <pre><code>xml_create = XMLCreateSerializer(data={'nome_emitente': 'NILVA', 'cnpj_destinatario':'5645654654','user_id': 1})
  
          if xml_create.is_valid():
              salvo = xml_create.save()
          else:
              salvo = xml_create.errors
  </code></pre>