Django ORM QuerySet is lazy — understanding evaluation triggers

Django queries execute at unexpected times or don't execute when expected. Database calls happen during template rendering instead of views.

QuerySets are lazy — no DB hit until evaluated. Triggers: iteration, slicing with step, len(), list(), bool(). Common mistakes: (1) .count() in loop instead of caching. (2) Filtering in template — use select_related/prefetch_related in view. (3) Multiple evaluations: cache with list(qs). Use django-debug-toolbar to see queries per request.

Lazy evaluation lets you chain filters without hitting the database. But actual SQL runs at consumption point, which can be surprising in template loops.