CODEWITHSUNDEEP
djangodjango-queryset
slumtrimpet
slumtrimpet

Reputation: 3267

How do I tell if a Django QuerySet has been evaluated?

I'm creating a Django queryset by hand and want to just use the Django ORM to read the resulting querset.query SQL itself without hitting my DB.

I know Django quersets are lazy and I see all the ops that trigger a queryset being evaluated:

https://docs.djangoproject.com/en/1.10/ref/models/querysets/#when-querysets-are-evaluated

But... what if I just want to verify my code is purely building the queryset guts but ISN'T evaluating and hitting my DB yet inadvertently? Are there any attributes on the queryset object I can use to verify it hasn't been evaluated without actually evaluating it?

Upvotes: 30

Views: 6158

Answers (1)

Peter DeGlopper
Peter DeGlopper

Reputation: 37319

For querysets that use a select to return lists of model instances, like a basic filter or exclude, the _result_cache attribute is None if the queryset has not been evaluated, or a list of results if it has. The usual caveats about non-public attributes apply.

Note that printing a queryset - although the docs note calling repr() as an evaluation trigger - does not in fact evaluate the original queryset. Instead, internally the original queryset chains into a new one so that it can limit the amount of data printed without changing the limits of the original queryset. It's true that it evaluates a subset of the original queryset and therefore hits the DB, so that's another weakness of this approach if you're in fact trying to use it to monitor all DB traffic.

For other querysets (count, delete, etc) I'm not sure there is a simple way. Maybe watch your database logs, or run in DEBUG mode and check connection.queries as described here: https://docs.djangoproject.com/en/dev/faq/models/#how-can-i-see-the-raw-sql-queries-django-is-running

Upvotes: 33

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