django-patterns

name	django-patterns
description	Use when editing Django models, views, serializers, admin, managers, signals, migrations, querysets, or database access patterns.

Django Patterns

Fat Models, Thin Views

• Put business logic in model methods, not views
• Views should handle HTTP concerns only: parse request, call model/service, return response
• Use model methods for validation, state transitions, and computed properties
• Use @property for derived attributes that don't need arguments

Custom Managers and QuerySets

• Use custom managers for reusable query logic (objects = MyManager())
• Define custom QuerySet classes and use as_manager() for chainable filters
• Common patterns: active(), for_user(user), with_related()
• Never put raw SQL in views -- encapsulate in manager methods

Query Optimization

• Always use select_related() for ForeignKey/OneToOne joins
• Always use prefetch_related() for ManyToMany/reverse FK
• Use only() / defer() to limit fetched columns when appropriate
• Use values() / values_list() for read-only aggregation queries
• Watch for N+1 queries in serializers and templates
• Use django-debug-toolbar or EXPLAIN to verify query plans

Signals

• Avoid signals for business logic -- prefer explicit method calls
• Acceptable uses: audit logging, cache invalidation, denormalization
• Always use dispatch_uid to prevent duplicate registration
• Keep signal handlers small and fast -- offload heavy work to tasks
• Document which signals exist and what they do

Migration Patterns

• One migration per logical change -- don't combine unrelated changes
• Use RunPython with both forward and reverse functions
• For large tables, consider batched data migrations
• Always test migrations with --plan before applying
• Use AddIndex / RemoveIndex separately from schema changes for zero-downtime
• Name migrations descriptively: 0042_add_user_email_verified_field

Admin Configuration

• Register all models with at least list_display and search_fields
• Use list_select_related to avoid N+1 in admin list views
• Use raw_id_fields for ForeignKey fields with many options
• Add readonly_fields for computed or audit fields