representing manytomanyfield with extra attributes in django - sql

Question

I have a situation where i need to store a table of "Product"'s and a table of "Order"'s on these products.
One order consists of many ("Product", "Quantity") tuples where quantity is a float expressed in tonnes.
I considered the followng implementation, but i don't think having a table of arbitrary products and quantities would be a very good design decision.
class Product(models.Model):
name = models.CharField("Name", max_length=50)
description = models.TextField("Description", blank=True)
class ProductOrder(models.Model):
unit = "tonnes"
product = models.ManyToManyField('Product')
quantity = models.FloatField('Quantity')
class Order(models.Model):
products = models.ManyToManyField('ProductOrder')
date = models.DateField('Date')
Am I overlooking an obvious solution? How would you implement this relationship to lead to the most DRY code. (PS. I don't want to have separate lists of products and quantities and have to rely implicitly on their ordering.)

Answer 1

In django, you can use through to create such intermediate table and maintain order specific attributes in there.
You can implement it as
class Product(models.Model):
name = models.CharField("Name", max_length=50)
description = models.TextField("Description", blank=True)
class Order(models.Model):
products = models.ManyToManyField('Product', through="OrderDetail")
date = models.DateField('Date')
class OrderDetail(models.Model):
unit = "tonnes"
product = models.ForeignKey('Product')
order = models.ForeignKey('Order')
quantity = models.FloatField('Quantity')
The documentation explains how to use and work with such design.