问题
I have a utility function in my program for searching for entities. It takes a max_count parameter. It returns a QuerySet.
I would like this function to limit the max number of entries. The standard way would be to take a slice out of my QuerySet:
return results[:max_count]
My problem is that the views which utilize this function sort in various ways by using .order_by(). This causes exceptions as re-ordering is not allowed after taking a slice.
Is it possible to force a "LIMIT 1000" into my SQL query without taking a slice?
回答1:
Do results[:max_count] in a view, after .order_by(). Don't be afraid of requesting too much from DB, query won't be evaluated until the slice (and even after it either).
回答2:
You could subclass QuerySet to achieve this, by simply ignore every slice and do [:max_count] at last in __getitem__, but I don't think it worths with the complex and side-effects.
If you are worrying about memory consumption by large queryset, follow http://www.mellowmorning.com/2010/03/03/django-query-set-iterator-for-really-large-querysets/
For normal usage, please just follow DrTyrsa's suggestion. You could write shortcuts to shorten the order_by and afterwards slice in code to simplify your code.
来源：https://stackoverflow.com/questions/9836659/providing-a-limit-param-in-django-query-without-taking-a-slice-of-a-queryset