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   |
|  |____________________________|   |       |________________________|
|                                   |
|___________________________________|

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.