Django admin: Inline straight to second-level relationship

Question

I have a three-levels Invoice model which I'd like to display on Django's admin area... in a "kind of special" way.

Allow me to provide a bit of background:

Each Invoice is conformed by several SubInvoice(s), and each SubInvoice is conformed by several InvoiceItem(s), which contain a break down of the Products purchased by a customer.

Logically speaking, it'd be something like this (hopefully the ascii art works)

+---------- Invoice id=3 -----------+
|       Full total: $100.00         |
|                                   |
|  +----- Sub Invoice id=1 -----+   |
|  |      Subtotal $70          |   |
|  |                            |   |
|  |    Item 1 in SubInv.1      |   |
|  |    Item 2 in SubInv.1      |   |
|  |    Item 3 in SubInv.1      |   |
|  |____________________________|   |
|                                   |
|  +----- Sub Invoice id=2 -----+   |
|  |      Subtotal $30          |   |
|  |                            |   |
|  |    Item 1 in SubInv.2      |   |
|  |    Item 2 in SubInv.2      |   |
|  |____________________________|   |
|                                   |
|___________________________________|

The models look more or less (they've been simplified for this question) like:

class Invoice(models.Model):
    full_total = DecimalField(...)
    # has a .sub_invoices RelatedManager through a backref from SubInvoice

class SubInvoice(models.Model):
    sub_total = DecimalField(...)
    invoice = ForeignKey('server.Invoice', related_name='sub_invoices')
    # has an .items RelatedManager through a backref from InvoiceItem

class InvoiceItem(models.Model):
    sub_invoice = ForeignKey('server.SubInvoice', related_name='items')
    product = ForeignKey('server.Product', related_name='+')
    quantity = PositiveIntegerField(...)
    price = DecimalField(...)

Now, I am aware that nesting two levels of relationships in Django Admin is very complex, and I'm not trying to nest the InvoiceItem into the SubInvoice and nest that one into the Invoice. That'd be great, but I'm ready to give that up due to the difficulties of nested inlines. No: what I'd like to to do is showing the Invoice and, as an inline, its Items, "jumping" through Invoice.sub_invoices__items. I don't really care that much about the information shown in the SubInvoice(s), but I do care about the information in the Invoice and in the InvoiceItems.

What I mean is that, basically, I would like (or "I could live with", rather) if the Invoice admin view looked like the following:

+---------- Invoice id=3 -----------+
|       Full total: $100.00         |
|                                   |
|  +----------------------------+   |
|  |                            |   |
|  |    Item 1 in SubInv.1      |   |
|  |    Item 2 in SubInv.1      |   |
|  |    Item 3 in SubInv.1      |   |
|  |    Item 1 in SubInv.2      |   |
|  |    Item 2 in SubInv.2      |   |
|  |____________________________|   |
|                                   |
|___________________________________|

(InvoiceItems as an inline of the Invoice(s) without showing any information about the SubInvoices in it)

I've tried the following in the admin.py:

class InvoiceItemInline(admin.StackedInline):
    fk_name = 'sub_invoice__invoice'
    model = InvoiceItem

class InvoiceAdmin(admin.ModelAdmin):
    inlines = (InvoiceItemInline,)

But that gives me an error:

<class 'server.admin.invoices.InvoiceItemInline'>: (admin.E202) 'server.InvoiceItem' has no field named 'sub_invoice__invoice'.

I've also tried directly this:

class InvoiceItemInline(admin.StackedInline):
    model = InvoiceItem

class InvoiceAdmin(admin.ModelAdmin):
    inlines = (InvoiceItemInline,)

But then (this one I was expecting) produces this error:

<class 'server.admin.invoices.InvoiceItemInline'>: (admin.E202) 'server.InvoiceItem' has no ForeignKey to 'server.Invoice'.

Is there any way of achieving this? Thank you in advance.

PS:

As of now, I have a "patched" solution which seems to be the canonical way:

• Register the Invoice model.
• Register an admin.ModelAdmin inline for the SubInvoice (this inline will be "inlined" into the Invoice's ModelAdmin).
• Also register the SubInvoice in the admin, so we can calculate a link to its admin view.
• Add an inline view of the InvoiceItems to the aforementioned SubInvoice's view.
• Add a link to the admin view of the SubInvoice(s) in the Invoice

Pretty much what is described in this other S.O. answer.

But the problem with this approach is that it won't let me see the Invoice and its InvoiceItemsat a glance (I see the invoice, with sub_invoices in it, and then within the sub_invoices inlines, there's a link to the InvoiceItems which I have to click on in order to see the items). It'd be great if I could get rid of the need for that link.

This is what I have now, basically:

+---------- Invoice id=3 -----------+
|       Full total: $100.00         |
|                                   |
|  +----- Sub Invoice id=1 -----+   |       +--- Sub Invoice id=1 ---+
|  |      Subtotal $70          |   |       |   Item 1 in SubInv.1   |
|  |                            |   |       |   Item 2 in SubInv.1   |
|  |    <a>Click for items ==============>  |   Item 3 in SubInv.1   |
|  |____________________________|   |       |________________________|
|                                   |
|  +----- Sub Invoice id=2 -----+   |
|  |      Subtotal $30          |   |       +--- Sub Invoice id=2 ---+
|  |                            |   |       |   Item 1 in SubInv.2   |
|  |    <a>Click for items ==============>  |   Item 2 in SubInv.2   |
|  |____________________________|   |       |________________________|
|                                   |
|___________________________________|

Blackfish

You can try django-nested-admin (github.com/theatlantic/django-nested-admin), which provides support for two-level inlines – Jonahjonas

Answer 1

I think your problem could be solved using the ManyToManyField + through. (This is an example)

#models.py
class Invoice(models.Model):
    full_total = DecimalField(...)
    # has a .sub_invoices RelatedManager through a backref from SubInvoice

class SubInvoice(models.Model):
    sub_total = DecimalField(...)
    invoice = ManyToManyField(
        'server.Invoice',
        through='server.InvoiceItem',
        through_fields=('sub_invoice', 'invoice'))
    # has an .items RelatedManager through a backref from InvoiceItem

class InvoiceItem(models.Model):
    sub_invoice = ForeignKey('server.SubInvoice')
    invoice = ForeignKey('server.Invoice')
    product = ForeignKey('server.Product', related_name='+')
    quantity = PositiveIntegerField(...)
    price = DecimalField(...)

#admin.py
from django.contrib import admin
from .models import InvoiceItem, Invoice, SubInvoice


class InvoiceItemInline(admin.TabularInline):
    model = InvoiceItem
    extra = 1


class InvoiceAdmin(admin.ModelAdmin):
    inlines = (InvoiceItemInline,)


admin.site.register(Invoice, InvoiceAdmin)
admin.site.register(SubInvoice, InvoiceAdmin)

I would recommend working with classes for this one in your views.py and use inlineformset_factory in your forms.py for this. Using jquery-formset library in your frontend as well, as this looks pretty neat together.

NOTE: You can also use on_delete=models.CASCADE if you want in the InvoiceItem on the foreignkeys, so if one of those items is deleted, then the InvoiceItem will be deleted too, or models.SET_NULL, whichever you prefer.

Hope this might help you out.

Atp

Thanks for the reply. I was trying to keep the InvoiceItem normalized (just with the sub_invoice FK, and not having to add the invoice also) – Blackfish

The Invoice isn't visible when you click on Invoice in the admin view, rather you will see all the SubInvoices with the InvoiceItems linked to the Invoice and in the SubInvoice you will see which Invoice and InvoiceItem it's linked to. Maybe you could do a test with this to see the results of what I mean? Because now you don't have to register the InvoiceItem in your admin.py to be able to see it. I would love to figure your problem out as I'm very much so interested in this :) – Atp

Sorry: I was actually talking about the models: In your code, the InvoiceItem must have two foreign keys: One to SubInvoice and another one to Invoice, which de-normalizes the tables since InvoiceItem.invoice is "redundant" (redundant in the sense that the invoice the InvoiceItem belongs to is also reachable through InvoiceItem --> sub_invoice --> invoice so in theory, I could force an InvoiceItem to belong to a subinvoice that doesn't actually belong to the same invoice, right? (maybe I'm looking at this wrong, though) – Blackfish

My concern (to further clarify the comment above) is: What happens if I stick the wrong value (let's say I put a... dunno... invoice_id=42) in InvoiceItem.invoice and then I have a situation in which, following the path InvoiceItem --> sub_invoice --> invoice I reach the invoice with ID 3, but following the path InvoiceItem --> invoice, I reach the invoice with ID 42 – Blackfish

In the example I gave you the SubInvoice has full_total, product, quantity and price. The Invoice has sub_total, product, quantity and price. In this case the SubInvoice, Invoice and InvoiceItem will all three still have use for each other. In your examples it seems like you mean to use this as well. Like this there also won't be any ID problems. I could post an image of how it looks in the database if you want to? – Atp

Yeah, those fields are used, and your example is fine in that regard: My concern comes because of the presence of the two FK (.invoice and .sub_invoice) in the InvoiceItem. Without InvoiceItem.invoice, the question What Invoice does this InvoiceItem belongs to? can uniquely be answered through InvoiceItem.sub_invoice.invoice But if I add the InvoiceItem.invoice FK, now I can (potentially) have 2 paths to reach the item's invoice: One, through InvoiceItem.sub_invoice.invoice and another one through InvoiceItem.invoice, no? If so, it feels it lead to potential bugs? – Blackfish

You can call to InvoiceItem.invoice and SubInvoice.through.invoice in this case, as for what Invoice the InvoiceItem belongs to is up to you which Invoice you want it to belong to. – Atp

You can however also have the ManyToManyField in the InvoiceItem refering to SubInvoice going through Invoice which will create a whole other scenario, like I said the code I wrote is just an example of how you can use a ManyToManyField, which I think can also do the thing that you want to establish. – Atp