Ordering many-to-many relations in Django models

Question

Suppose you have a many-to-many relation in a Django model, such as:

class GroceryList(models.Model):
    items = models.ManyToManyField(GroceryItem, related_name='in_lists')

class GroceryItem(models.Model):
    name = models.CharField(unique=True)

You and me can both have the same items in two different lists, such as Avocado, and they will point to the same Avocado object.

What is the best way to implement an arbitrary order for items in each list, that can be edited separately for each list? (i.e. I have Avocado first in my list, while you have it at index 4)

django-ordered-model seems like an interesting solution, but it assumes global order across all objects.

Answer 1

You can use the intermediate table using through and add the ordered field in that table.

class GroceryList(models.Model):
    items = models.ManyToManyField(GroceryItem, related_name='in_lists', 
            through='Order')

class GroceryItem(models.Model):
    name = models.CharField(unique=True)

class Order(models.Model):
    number = models.PositiveIntegerField()
    gl = models.ForeignKey(GroceryList)
    gi = models.ForeignKey(GroceryItem)

So instead of doing grocerylist.items.add(groceryitem) you can do

#for groceryitem1 as 1st item in grocerylist1
Order.objects.create(gl=grocerylist1, gi=groceryitem1, number=1)

#for groceryitem1 as 10th item in grocerylist2
Order.objects.create(gl=grocerylist2, gi=groceryitem1, number=10)