django-orm-patterns

name

django-orm-patterns

description

Django ORM patterns: model definitions, field types, model managers, custom QuerySet methods, select_related/prefetch_related for N+1 prevention, transactions, F/Q expressions, Meta indexes and constraints. For django-architect (model definitions) and django-migrations-specialist (field finalization). Activated automatically by django-plugin/stack.md. Use this skill to: - Write clean, efficient Django model definitions with proper field choices and __str__. - Use custom managers and QuerySets to encapsulate query logic. - Prevent N+1 queries with select_related and prefetch_related. - Use transactions for atomic operations. - Define Meta indexes and constraints (for django-migrations-specialist to finalize). Do NOT use this skill for: - Creating actual migrations (makemigrations) — that is django-migrations-specialist. - Web view / API patterns — see django-plugin:django-conventions. - Python idioms — see python-foundation:python-conventions.

Django ORM Patterns

This skill covers Django ORM model definitions, query patterns, and database-level options. Apply alongside django-plugin:django-conventions when implementing features.

Role split:

• django-architect uses this skill for model definitions — fields, __str__, choices, class Meta ordering, custom managers, and query patterns.
• django-migrations-specialist uses this skill for field finalization — setting max_digits, on_delete, db_index, Meta.indexes, Meta.constraints — before running makemigrations.

1. Detection

Check the Django version in requirements.txt or pyproject.toml before writing constraint code — Meta.constraints using CheckConstraint uses condition= in Django 5.1+ and check= in Django 4.x. When uncertain, check the installed version with python manage.py --version or read the pinned version from the dependency file.

2. Model definition patterns

Use TextChoices for string-typed status/type fields. Write a meaningful __str__. Always specify class Meta ordering.

from django.db import models
from django.contrib.auth import get_user_model

User = get_user_model()


class Order(models.Model):
    class Status(models.TextChoices):
        PENDING = 'pending', 'Pending'
        PROCESSING = 'processing', 'Processing'
        SHIPPED = 'shipped', 'Shipped'
        CANCELLED = 'cancelled', 'Cancelled'

    user = models.ForeignKey(User, on_delete=models.PROTECT, related_name='orders')
    product = models.CharField(max_length=255)
    qty = models.PositiveIntegerField()
    unit_price = models.DecimalField(max_digits=10, decimal_places=2)
    status = models.CharField(max_length=20, choices=Status.choices, default=Status.PENDING)
    notes = models.TextField(blank=True, default='')
    created_at = models.DateTimeField(auto_now_add=True)
    updated_at = models.DateTimeField(auto_now=True)

    class Meta:
        ordering = ['-created_at']
        indexes = [
            models.Index(fields=['status', 'created_at']),
        ]
        constraints = [
            models.UniqueConstraint(fields=['user', 'reference'], name='unique_user_reference'),
            # Django 5.1+: condition=; Django 4.x: check=
            models.CheckConstraint(condition=models.Q(qty__gt=0), name='orders_order_qty_positive'),
        ]

    def __str__(self) -> str:
        return f'Order #{self.pk} — {self.product} ({self.get_status_display()})'

    @property
    def is_cancellable(self) -> bool:
        return self.status in (self.Status.PENDING, self.Status.PROCESSING)

3. Custom managers and QuerySets

Encapsulate common query logic in a custom QuerySet and expose it via a manager. Use as_manager() for the shortcut form when you don't need manager-specific methods.

from django.db import models


class OrderQuerySet(models.QuerySet):
    def active(self) -> 'OrderQuerySet':
        return self.exclude(status=Order.Status.CANCELLED)

    def by_user(self, user) -> 'OrderQuerySet':
        return self.filter(user=user)

    def pending(self) -> 'OrderQuerySet':
        return self.filter(status=Order.Status.PENDING)


class OrderManager(models.Manager):
    def get_queryset(self) -> OrderQuerySet:
        return OrderQuerySet(self.model, using=self._db)

    def active(self) -> OrderQuerySet:
        return self.get_queryset().active()

    def for_dashboard(self, user) -> OrderQuerySet:
        return self.get_queryset().by_user(user).active().select_related('user')


class Order(models.Model):
    objects = OrderManager()
    # shortcut form (no custom manager methods needed):
    # objects = OrderQuerySet.as_manager()
    ...

Usage: Order.objects.active(), Order.objects.for_dashboard(request.user).

4. N+1 prevention

Always load related objects in advance when iterating. The most common Django performance problem is accessing a FK in a loop without prefetching.

select_related — SQL JOIN for FK/OneToOne

# Bad: triggers 1 query per order to fetch user
orders = Order.objects.all()
for order in orders:
    print(order.user.email)  # N+1

# Good: single JOIN query
orders = Order.objects.select_related('user').all()

prefetch_related — separate query for ManyToMany / reverse FK

# Prefetch all items for each order in a single extra query
orders = Order.objects.prefetch_related('items').all()

# Custom prefetch with its own queryset
from django.db.models import Prefetch

orders = Order.objects.prefetch_related(
    Prefetch('items', queryset=OrderItem.objects.select_related('product'))
)

only() and defer() for field projection

# Fetch only the columns you need
orders = Order.objects.only('id', 'status', 'created_at')

# Defer a large column (e.g., rich text body)
articles = Article.objects.defer('body')

5. F expressions and Q objects

F() — reference a field value in the database

from django.db.models import F

# Atomic increment — no race condition
Order.objects.filter(pk=pk).update(qty=F('qty') + 1)

# Compare two fields
Order.objects.filter(shipped_at__lt=F('created_at'))

Q() — complex OR / AND queries

from django.db.models import Q

# OR
Order.objects.filter(Q(status='pending') | Q(status='processing'))

# NOT
Order.objects.filter(~Q(status='cancelled'))

# AND + OR combined
Order.objects.filter(
    Q(user=request.user) & (Q(status='pending') | Q(status='processing'))
)

annotate() with aggregates

from django.db.models import Count, Sum, Avg

# Count orders per user
from apps.users.models import User
users = User.objects.annotate(order_count=Count('orders'))

# Sum order totals per user
users = User.objects.annotate(
    total_spend=Sum(F('orders__qty') * F('orders__unit_price'))
)

6. Transactions

Use @transaction.atomic or the context manager form to wrap operations that must succeed or fail together.

from django.db import transaction


@transaction.atomic
def place_order(user, product: str, qty: int) -> Order:
    order = Order.objects.create(user=user, product=product, qty=qty)
    inventory = Inventory.objects.select_for_update().get(product=product)
    if inventory.stock < qty:
        raise ValueError("Insufficient stock")
    inventory.stock = F('stock') - qty
    inventory.save(update_fields=['stock'])
    return order

transaction.on_commit() — run after the transaction commits

Use this to enqueue async tasks. Never enqueue a task from inside an atomic block directly — if the transaction rolls back, the task will still run.

from django.db import transaction

def place_order(user, product: str, qty: int) -> Order:
    with transaction.atomic():
        order = Order.objects.create(user=user, product=product, qty=qty)
        transaction.on_commit(lambda: send_confirmation_email.delay(order.pk))
    return order

7. Meta indexes and constraints

These are defined in model code by django-architect and finalized (verified, regenerated after version check) by django-migrations-specialist.

class Meta:
    indexes = [
        models.Index(fields=['status', 'created_at']),         # composite index for common filter
        models.Index(fields=['user'], name='orders_user_idx'),  # explicit name
    ]
    constraints = [
        models.UniqueConstraint(
            fields=['user', 'reference'],
            name='unique_order_reference_per_user',
        ),
        # Django 5.1+: condition=  |  Django 4.x: check=
        models.CheckConstraint(
            condition=models.Q(qty__gt=0),
            name='orders_order_qty_must_be_positive',
        ),
    ]

Name conventions: <app>_<model>_<description> — keeps names unique and self-documenting.

8. Anti-patterns

Don't	Do
.filter(...) inside a loop	Batch with __in: Order.objects.filter(pk__in=ids)
Access FK attribute in a loop without prefetch	Use select_related (FK/1:1) or prefetch_related (M2M/reverse FK)
Update many objects one by one with .save()	QuerySet.update(status='shipped') for bulk updates
.save() for single-field updates	obj.save(update_fields=['status']) — avoids overwriting concurrent writes
Order.objects.all() then filter in Python	Push filtering to the DB with .filter() / .exclude()
Raw SQL with f-strings: .raw(f'... {user_id}')	Parameterized: .raw('... WHERE id = %s', [user_id]) or use ORM