Django lazy QuerySet and pagination

问题:

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.

回答1:

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.

回答2:

It is. Django's pagination uses the same rules/optimizations that apply to querysets.

This means it will start evaluating on return HttpResponse(output)