title: Unique foreign key pairs with Django
body: <p>I have three models: <strong>products</strong>, <strong>users</strong>, and <strong>reviews</strong>.</p>
  
  <p>A review is linked to a product and a user as follows:</p>
  
  <pre><code>class Review(models.Model):
      product = models.ForeignKey(Product)    
      user = models.ForeignKey(User)
      review_text = models.TextField()
      creation_date = models.DateTimeField(auto_now_add=True)
  </code></pre>
  
  <p>I'd like to allow each user to submit only one review per product. What is the recommended way to achieve this? Through the model, through verification, or something else? I'm very new to Django/Python. Thanks.</p>