Django Schema Design

Django Schema Designer

Design production-ready Django schemas. The value here is PK strategy and the non-obvious trade-offs behind it — not basic ORM syntax.

Quick Start

Describe your data model in Django terms:

design Django models for invoices, line items, and customers

Include in your request:

• Entities and relationships
• Whether IDs are exposed in URLs/APIs (security)
• Whether rows are inserted at high volume and read in creation order (audit, events, feeds)
• Scale hints (optional)

Triggers

Before you model, interrogate the domain

Fields are the easy part; the schema's shape is decided by four questions asked before any field exists:

1. Identity — what makes a row the same row? If a natural key exists (email, ISO code, (tenant, slug)), enforce it as a UniqueConstraint even when you keep a surrogate PK. A schema with no uniqueness rule other than the auto-PK will silently accept duplicates.
2. Cardinality & ownership — for each relationship, which side owns the lifecycle? That answer is your on_delete (PROTECT for accounting/audit rows you must never lose, CASCADE for true children, SET_NULL for optional links).
3. Lifecycle — is a row mutable, append-only, or soft-deleted? Append-only tables (events, audit) want UUIDv7 and no updated_at; soft-deleted tables need a partial unique index (below), not a plain unique=True.
4. Access paths — what are the 2–3 queries that run most? Those dictate indexes. Design indexes for reads you have, not reads you imagine.

Primary Key Strategy

Choose one PK type per model. The decision hinges on two axes experts weigh and beginners miss: enumeration exposure and index-insert locality.

Decision flow (principle, not rule):

1. ID exposed in API/URL, or the entity is a sensitive target (User, Tenant, token)? → UUID family. Go to 2.
2. High insert rate, or you read rows in creation order? → UUIDv7. Else → UUIDv4.
3. None of the above (internal, generic)? → BigAutoField.

The non-obvious reason UUIDv7 exists

A v4 UUID is random, so every insert lands at a random point in the primary-key B-tree. On a hot table this fragments the index, thrashes the buffer cache, and inflates write amplification — the classic "UUID PK killed our insert throughput" outcome.

UUIDv7 is time-ordered: new keys sort to the right of the tree, so inserts stay sequential like an integer PK — you keep unpredictability and insert locality. It also lets you ORDER BY id as a proxy for creation time, removing the need for a separate index on created_at for feed/audit queries.

So the real axis isn't "unique vs not" (all three are unique) — it's random placement (v4) vs sequential placement (v7/incremental).

Examples (inspiration, not rules):

• Invoices → incremental — internal id; the human-facing invoice number is a separate unique field.
• Users / API keys → UUIDv4 — enumeration and IDOR must be impossible; insert rate is low, so v4's locality cost is irrelevant.
• AuditLog / Events → UUIDv7 — unpredictable, hot inserts, read newest-first.

Details, storage/collation notes, and PK migration: references/pk-strategy.md.

NEVER

• NEVER use FloatField for money — binary floats can't represent 0.10 exactly; you get silent rounding drift. Use DecimalField(max_digits=..., decimal_places=2).
• NEVER put a sequential integer PK on a user-facing or sensitive entity — id=1, 2, 3 in a URL invites enumeration and IDOR (walk from /users/1/ upward). Use a UUID.
• NEVER reach for UUIDv4 as the PK of a high-insert table you read in creation order — random placement fragments the index; use UUIDv7 for insert locality.
• NEVER switch a live table's PK in a single migration — add-backfill-repoint-swap across separate, reversible migrations (see references).
• NEVER add a ForeignKey without an explicit on_delete — silent CASCADE deletes data; state intent (PROTECT, SET_NULL, CASCADE) every time.

UUIDv7 Field (the one snippet worth keeping)

Requires PostgreSQL with a uuid_generate_v7() function (e.g. the pg_uuidv7 extension) and Django 5.0+ for db_default.

from django.db import models
from django.db.models.expressions import RawSQL


class UUIDv7Field(models.UUIDField):
    """Time-ordered UUID PK generated in the database."""

    def __init__(self, *args, **kwargs):
        kwargs.setdefault("primary_key", True)
        kwargs.setdefault("editable", False)
        kwargs.setdefault("db_default", RawSQL("uuid_generate_v7()", []))
        super().__init__(*args, **kwargs)


class AuditLog(models.Model):
    id = UUIDv7Field()
    actor_id = models.UUIDField(null=True, blank=True)
    action = models.CharField(max_length=64)
    created_at = models.DateTimeField(auto_now_add=True)
    # No index on created_at needed: ORDER BY id is chronological.

Incremental (BigAutoField) and plain UUIDv4 (UUIDField(default=uuid.uuid4, editable=False)) PKs are standard Django — write them directly; no snippet needed.

Indexes & Constraints (the expert layer)

The non-obvious wins live here, not in the field types.

Composite index column order is not free. An index on (a, b) also serves queries on a alone, but not on b alone. Order columns by selectivity/most-common-filter first. A WHERE tenant_id=? AND status=? query wants Index(fields=["tenant_id", "status"]) — reversing it wastes the index for the tenant-only query you also run.

class Order(models.Model):
    class Meta:
        indexes = [
            # Covers "tenant's open orders newest-first" in one index scan.
            models.Index(fields=["tenant_id", "status", "-created_at"]),
        ]

Partial unique indexes beat unique=True for soft-delete and nullable-scoped uniqueness. "Email unique among non-deleted users" is impossible with unique=True; use a condition:

class Meta:
    constraints = [
        models.UniqueConstraint(
            fields=["email"],
            condition=models.Q(deleted_at__isnull=True),
            name="uniq_active_email",
        ),
    ]

Push invariants into CheckConstraint, not just app code. The DB is the only layer that can't be bypassed by a stray script or shell:

models.CheckConstraint(check=models.Q(quantity__gt=0), name="qty_positive")

FK indexing: Django auto-indexes single-column FKs — do not re-declare db_index=True on them (duplicate index). Do add indexes for FKs you filter in combination with other columns (composite, above).

Model the FK design for the query, not just the diagram: a deep FK chain you always traverse means select_related (SQL join); a reverse/many relation you fan out over means prefetch_related (second query). If a hot read forces 4 joins, that's a signal to denormalize a column, not to add a 4th index.

Covering (include) indexes, ExclusionConstraint, GIN/GiST/BRIN selection, and reading EXPLAIN: references/indexing-and-constraints.md.

Migrations (the parts that bite)

This is the low-freedom part — a wrong migration locks a table or drops data. Follow the sequence, don't improvise.

• Adding a NOT NULL column to a populated table: add nullable → data-migration backfill → alter to NOT NULL. One shot fails on existing rows.
• Backfill in batches on large tables — a single UPDATE over millions of rows takes a lock and bloats the transaction; iterate with .filter(pk__gt=last).order_by("pk")[:batch].
• Index creation on a live table must be concurrent — a plain CREATE INDEX holds a write lock for the whole build. Use AddIndexConcurrently (from django.contrib.postgres.operations) with atomic = False on the migration.
• Reversibility: pair every RunPython with reverse_code; a migration you can't roll back is a migration you can't deploy safely.
• Changing a live PK: never in-place. See the staged add→backfill→repoint-FKs→swap procedure in references/pk-strategy.md.

Verification Checklist

• Every model has a deliberate PK choice (incremental / UUIDv4 / UUIDv7), justified by exposure + insert pattern.
• No sequential PK on any URL-exposed or sensitive entity.
• Every ForeignKey/OneToOneField has an explicit on_delete reflecting lifecycle ownership.
• Natural keys enforced via UniqueConstraint (partial where soft-delete/nullable-scoped applies).
• Domain invariants pushed into CheckConstraint, not only app code.
• Composite indexes ordered by most-common-filter first; no duplicate db_index on single FKs.
• Monetary values use DecimalField, never FloatField.
• created_at/updated_at where relevant.
• Migrations reversible, backfilled in batches, indexes created concurrently on live tables.

References

Load on demand — each file has a distinct trigger. Loading both for a simple task wastes context.

Do NOT load either for straightforward model or field additions.