Django lazy QuerySet and pagination

django, django-queryset, lazy-loading

Solution

If you want to see where are occurring, import `django.db.connection` and inspect `queries`

>>> from django.db import connection
>>> from django.core.paginator import Paginator
>>> queryset = Entry.objects.all()

Lets create the paginator, and see if any queries occur:

>>> paginator = Paginator(queryset, 10)
>>> print connection.queries 
[]

None yet.

>>> page = paginator.page(4)
>>> page
<Page 4 of 788>
>>> print connection.queries 
[{'time': '0.014', 'sql': 'SELECT COUNT(*) FROM `entry`'}]

Creating the page has produced one query, to count how many entries are in the queryset. The entries have not been fetched yet.

Assign the page's objects to the variable 'objects':

>>> objects = page.object_list
>>> print connection.queries
[{'time': '0.014', 'sql': 'SELECT COUNT(*) FROM `entry`'}]

This still hasn't caused the entries to be fetched.

Generate the `HttpResponse` from the object list

>>> response = HttpResponse(page.object_list)
>>> print connection.queries
[{'time': '0.014', 'sql': 'SELECT COUNT(*) FROM `entry`'}, {'time': '0.011', 'sql': 'SELECT `entry`.`id`, <snip> FROM `entry` LIMIT 10 OFFSET 30'}]

Finally, the entries have been fetched.

Problem

I read here that Django querysets are lazy, it won't be evaluated until it is actually printed. I have made a simple pagination using the django's built-in pagination. I didn't realize there were apps already such as "django-pagination", and "django-endless" which does that job for. Anyway I wonder whether the QuerySet is still lazy when I for example do this ``` entries = Entry.objects.filter(...) paginator = Paginator(entries, 10) output = paginator.page(page) return HttpResponse(output) ``` And this part is called every time I want to get whatever page I currently I want to view. I need to know since I don't want unnecessary load to the database.

Original source