Prefixed ULID Primary Keys

This project uses Stripe-style prefixed ULIDs as the primary key for every Django model:

prd_01jq3v8f6a7b2c8d9e0f1g2h3j4k
ord_01jq3v8fgh7x2y5z9a1b2c3d4e5f

A 3-4 character prefix identifies the entity type, followed by an underscore and a lowercase ULID. ULIDs are 128-bit, lexicographically sortable by creation time, URL-safe, and collision-resistant.

Why

• Debuggable. ord_01jq... in a log line tells you immediately it's an order — no need to cross-reference the column.
• Safe to expose. Unlike auto-increment integers, prefixed ULIDs leak no ordering or volume information, and unlike opaque UUIDs they remain human-readable.
• Time-sortable. ULIDs sort chronologically, so ORDER BY id doubles as ORDER BY created_at without a second index.
• Type-safe across layers. Every ID is a str end-to-end — no UUID / str coercion at the service/API boundary.
• No integer collisions. Exporting, importing, and sharding are all easier without monotonic counters.

The Generator

Put this in src/project/ids.py:

from ulid import ULID


def prefixed_ulid(prefix: str) -> str:
    return f"{prefix}_{str(ULID()).lower()}"


def _make_generator(prefix: str):
    def generate() -> str:
        return prefixed_ulid(prefix)

    generate.__name__ = f"generate_{prefix}_id"
    generate.__qualname__ = f"generate_{prefix}_id"
    return generate

Then register a generator per aggregate root:

generate_prd_id = _make_generator("prd")
generate_ord_id = _make_generator("ord")
generate_itm_id = _make_generator("itm")

The __name__ / __qualname__ rewrite matters: Django migrations serialize the default callable's fully qualified name, so each generator needs a distinct identity or the autodetector will get confused.

Choosing a Prefix

• 3 to 4 lowercase letters — short enough to stay readable in logs
• Must be unique across the whole project
• Prefer mnemonic, not cryptic: ord for order, inv for invoice, prd for product, usr for user
• Avoid collisions with existing prefixes — grep src/project/ids.py before inventing a new one
• Never rename a prefix once it's in production; the prefix is part of the ID

Using it in a Model

from typing import ClassVar

from django.db import models

from project.ids import generate_prd_id


class Product(models.Model):
    __prefix__: ClassVar[str] = "prd"
    id = models.CharField(
        max_length=64,
        primary_key=True,
        default=generate_prd_id,
        editable=False,
    )
    name = models.CharField(max_length=255)

    def __str__(self) -> str:
        return self.name

Rules:

• CharField(max_length=64) — ULID is 26 chars, prefix + separator adds up to ~10, 64 leaves headroom.
• primary_key=True and editable=False.
• The default is the generator function (no parentheses) so Django calls it per row.
• __prefix__: ClassVar[str] mirrors the generator's prefix — makes the mapping discoverable from the model class alone, and lets tests assert on it.
• Never override save() to generate the ID; the default handles it.

Using it Across the Stack

Once IDs are strings at the ORM layer, they stay strings everywhere else:

• DTOs (Pydantic): id: str — never UUID.
• Repository params: def get(self, product_id: str) -> ProductDTO: ...
• API path params (django-ninja): def get_product(request, product_id: str): ...
• Celery task args: pass the string ID, never a model instance.
• Tests: assert on the prefix, e.g. assert dto.id.startswith("prd_").

The prefix is also a cheap sanity check on every boundary: if an ID ever shows up without its prefix, something has stripped or regenerated it incorrectly.

Migrating an Existing Table

If the table already has integer or UUID primary keys, don't try to change them in place. Instead:

1. Add a new CharField column with the prefixed ULID default, nullable at first.
2. Backfill with a data migration that assigns prefixed_ulid("prd") to every existing row.
3. Add unique=True and make it non-nullable in a follow-up migration.
4. Swap it to primary_key=True only after every foreign key has been migrated to reference the new column — this usually means a multi-release cutover.

Prefer doing this on a new table where possible; in-place primary-key swaps in production are a lot of work for limited benefit.

Verify

• Every model has __prefix__ and a CharField primary key using a generate_*_id default.
• Every generate_*_id in src/project/ids.py has a unique prefix.
• No model uses UUIDField, AutoField, or BigAutoField for its primary key.
• No DTO field, service argument, or API path param types an ID as UUID — they're all str.

uv run ruff check src
uv run pyrefly check src
uv run pytest