QuerySets

Both Invoice and Item use custom QuerySet managers, accessible via Invoice.objects and Item.objects.

InvoiceQuerySet

All methods return a filtered QuerySet and can be chained.

Status / payment filters

Method	Returns
.paid()	Invoices with status=PAID
.unpaid()	Invoices that are not paid, cancelled, credited, and have a non-zero total
.overdue()	Unpaid invoices whose date_due is in the past (excludes credit notes)
.not_overdue()	Invoices that are not overdue (either future due date, or in a terminal status)
.in_collection()	Invoices with status=IN_COLLECTION
.not_in_collection()	Invoices with any status other than IN_COLLECTION

Validity / accounting filters

Method	Returns
.valid()	Excludes RETURNED, CANCELED, CREDITED, and UNCOLLECTIBLE invoices
.accountable()	Excludes proforma and advance invoices (not yet legally binding)
.collectible()	Excludes UNCOLLECTIBLE invoices
.uncollectible()	Only UNCOLLECTIBLE invoices

Origin filters

Method	Returns
.issued()	Only origin=ISSUED invoices
.received()	Only origin=RECEIVED invoices

Relation filters

Method	Returns
.having_related_invoices()	Invoices that have at least one related invoice
.not_having_related_invoices()	Invoices with no related invoices

Utilities

.duplicate_numbers()

Returns a list of invoice number strings that appear more than once in the queryset. Useful for data integrity checks.

dupes = Invoice.objects.duplicate_numbers()
if dupes:
    print(f"Duplicate numbers found: {dupes}")

.lock()

Acquires a SHARE ROW EXCLUSIVE table-level lock (PostgreSQL). Called internally by the sequence generator to prevent race conditions during sequence assignment. Silently no-ops on SQLite and other backends that do not support LOCK TABLE.

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ItemQuerySet

Method	Returns
.with_tag(tag)	Items whose tag field equals the given string

invoice.item_set.with_tag('shipping')