Edit django 'through' model inline in wagtail admin? - django

Question

[Edited with better code sample]
As per the title I am trying to allow for inline editing for a very simple shop page in Wagtail (will probably make it into a simple package):
With the following models:
class Product(ClusterableModel):
page = ParentalKey(MiniShopPage, on_delete=models.CASCADE, related_name='shop_products')
name = models.CharField(max_length=255)
description = models.CharField(max_length=2500)
downloadable = models.BooleanField()
price = models.FloatField()
image = models.ForeignKey(
'wagtailimages.Image',
null=True,
blank=True,
on_delete=models.SET_NULL,
related_name='+'
)
# define the content_panels
panels = [
FieldPanel('name'),
FieldPanel('description'),
FieldPanel('downloadable'),
FieldPanel('price'),
ImageChooserPanel('image'),
]
class Order(TimeStampedModel, ClusterableModel):
'''
Example of use outside of the admin:
p = Product.objects.first()
order = Order.objects.create(client_email='someone#hotmail.com', gift_amount=0)
quantities = ProductInOrderCount(product=p, order=order, quantity=2)
quantities.save()
for itm in order.productinordercount_set.all():
print(itm.quantity)
'''
is_fulfilled = models.BooleanField(default=False)
is_paid_for = models.BooleanField(default=False)
client_email = models.EmailField(blank=False)
gift_amount = models.PositiveIntegerField()
# products = M2MTHROUGH
# the through model stores the quantity
products = models.ManyToManyField(Product, through='ProductInOrderCount')
content_panels = [
FieldPanel('is_fulfilled'),
FieldPanel('is_paid_for'),
FieldPanel('client_email'),
FieldPanel('gift_amount'),
InlinePanel('products'),
]
class OrderModelAdmin(ModelAdmin):
model = Order
menu_label = 'Orders'
...
modeladmin_register(OrderModelAdmin)
class ProductInOrderCount(Orderable):
product = models.ForeignKey(Product, on_delete=models.CASCADE)
order = models.ForeignKey(Order, on_delete=models.CASCADE)
quantity = models.PositiveIntegerField()
The tricky thing is that I get the error Cannot set values on a ManyToManyField which specifies an intermediary model. Or I simply don't get an inline panel, but rather a select.
I am assuming this is the case because the create and add methods do not work on through models, is that the case?
If so could you suggest a way I can rewrite the app so as to allow me to create orders with products in the admin and in my code?

Answer 1

InlinePanel only works with one-to-many ParentalKey relations, not a ManyToManyField. That shouldn't be a problem, because ParentalKey is a good fit for this case:
A ManyToManyField with a through model is really just two one-to-many relations back to back;
ParentalKey is designed for relations that are closely tied to the parent model, in the sense that they're always edited, validated and saved as a single unit. This is true for the relation between ProductInOrderCount and Order (a ProductInOrderCount record is conceptually part of an Order), but not the relation between ProductInOrderCount and Product (a ProductInOrderCount is not part of a Product).
This would give you a model definition like:
class ProductInOrderCount(Orderable):
order = ParentalKey(Order, on_delete=models.CASCADE, related_name='ordered_products')
product = models.ForeignKey(Product, on_delete=models.CASCADE)
quantity = models.PositiveIntegerField()
Your Order model can then have an InlinePanel('ordered_products'), and the products field can be omitted.