.any() method on QuerySet using a known trick

[Vlada Macek]

While inspecting the Django code, I found this trick:

# This cute trick with extra/values is the most efficient way to   
# tell if a particular query returns any results.                  
if qs.extra(select={'a': 1}).values('a').order_by():

It apparently turns off the ordering as well as retrieving and parsing any real value from the db, resulting in something very cheap like:

SELECT (1) AS "a" FROM <table> WHERE <previous-qs-filtering>

Which is what we wanted to get the any/none info.

I think many of us frequently need this, but build unnecessarily complex constructs, while this can easily make it to a new QuerySet method called, say, .any().

[Russell Keith-Magee]

Duplicate of #11402