title: NOT NULL constraint failed: product_product.author_id
body: <p>When I am going to post objects in the postman I am getting this error</p>
  
  <blockquote>
    <p>NOT NULL constraint failed: product_product.author_id</p>
  </blockquote>
  
  <p>I included Basic Auth in the Authorization section anyway it gives me error.</p>
  
  <p><strong>models.py</strong></p>
  
  <pre><code>class Product(models.Model):
  category = models.ForeignKey(Category, on_delete=models.CASCADE)
  name = models.CharField(max_length=200)
  brand = models.CharField(max_length=200)
  rating = models.PositiveSmallIntegerField(default=0)
  description = models.TextField()
  author = models.ForeignKey(User, on_delete=models.CASCADE)
  </code></pre>
  
  <p><strong>serializers.py</strong></p>
  
  <pre><code>class ProductSerializer(serializers.HyperlinkedModelSerializer):
  category = serializers.SlugRelatedField(queryset=Category.objects.all(), slug_field='name')
  author = serializers.ReadOnlyField(source='author.username')
  class Meta:
      model = Product
      fields = ['id', 'url', 'category', 'name', 'brand', 'rating', 'description', 'price']
  </code></pre>
  
  <p>Why Not Null constraint happens? how can I solve this? Thanks in advance!</p>