title: Django - Many to many form field + extra "through" fields
body: <p>I've been looking for a solution to my problem for several days but I can't seem to find exactly what I want. I'd like to make that shopping-cart-like form to book some products in a list. You would check the products that you need and fill in the amount for each one. A reservation can be edited afterwards.</p>
  
  <p>My problem is that reservations and products are linked through a many-to-many intermediary model to have an extra "quantity" field :</p>
  
  <pre><code>class Reservation(models.Model):
      # some fields...
      products = models.ManyToManyField(Product, through='ProductReservation')
  
  class Product(models.Model):
      # some fields...
  
  class ProductReservation(models.Model):
      quantity = models.IntegerField()
      reservation = models.ForeignKey(Reservation)
      product = models.ForeignKey(Product)
  </code></pre>
  
  <p>I have seen some posts dealing with a similar issue in the admin but here I need to make it available to the client. I have thought about using an inline formset with a custom queryset, but I couldn't manage to make the link between the different classes. I could always populate my fields directly from the submitted data, but I need to reuse that form in multiple views and anyway that doesn't sound very DRY to me.</p>
  
  <p>Thanks for your help !</p>