Model Classes

object_id can hold a key from any table, so the database cannot declare a constraint on it; integrity, cascades and joins move into Django. For a few fixed targets, nullable ForeignKeys with a CheckConstraint, or per-target tables, restore both.

Use a UUID key when ids must be unguessable or created outside the database. It costs a wider key in every index and foreign key, char(32) off PostgreSQL and MariaDB, and random uuid4 values scatter inserts across the index; time-ordered UUIDv7 avoids that.

Each child save writes one row per level inside a transaction, bulk_create() raises ValueError for multi-table children, bulk_update() adds a query per ancestor, deleting a child also deletes its parent row unless keep_parents=True, and select_for_update(of=...) must name the parent link.

An instance holds the values read when it was loaded; QuerySet.update(), other processes and raw SQL change the row without touching it. refresh_from_db() reloads its fields. Since Django 6.0, F() assignments are refreshed automatically on save().

db_index=True gives one plain single-column index. Meta.indexes with models.Index adds multi-column, descending, partial (condition), covering (include) and functional (expressions) indexes. Those need a name, and some backends ignore condition or include.

PROTECT refuses any delete that reaches a protected row. RESTRICT refuses it too, except when the restricted rows are themselves being deleted through a CASCADE path in the same operation; then the delete proceeds.

Classic CASCADE runs in Python: Django's deletion collector finds the related rows and deletes them itself, sending delete signals. Django 6.1's DB_CASCADE writes ON DELETE CASCADE into the foreign key instead, so the database deletes children without Django fetching them or sending signals.

Assigning None to a JSONField stores SQL NULL; a JSON null needs Django 6.1's JSONNull() expression, and both read back as None. Its default must be a callable such as dict, or all instances share one mutable object.

Declare pk = models.CompositePrimaryKey('order_id', 'line_no'); pk becomes a tuple usable in filters. Limits: ForeignKey and generic relations cannot target the model, the admin cannot register it, and Django cannot migrate a table to or from it.

A child of an abstract base inherits the whole Meta if it declares none, with abstract reset to False, and can extend it by subclassing Base.Meta; a multi-table or proxy child inherits only ordering and get_latest_by.

Each content_object read runs its own query. prefetch_related('content_object') groups likes by content type and runs one query per target model; GenericPrefetch, new in Django 5.0, supplies a custom queryset per type, such as one using select_related or only().

Write a Transform subclass with lookup_name = 'currency', an SQL template extracting the code and output_field = CharField(), then register it with MoneyField.register_lookup(). A Transform yields a value, so exact, in and order_by chain after it.