id	summary	reporter	owner	description	type	status	component	version	severity	resolution	keywords	cc	stage	has_patch	needs_docs	needs_tests	needs_better_patch	easy	ui_ux
21422	prefetch_related does not document restriction of requiring homogeneous results	pab@…	Tim Graham <timograham@…>	"I have just been trying to track down a performance issue in my application using Django 1.6.  The solution was to use prefetch_related(), bit it didn't quite work as expected.  The documentation was great as far as it went and so I found the necessary function with little fuss - well done!  However it didn't warn me that prefetch_related() must be restricted to a homogenous set of results.

In particular, I had query that needed to fetch data for objects found through a GenericForeignKey and then a ManyToManyField.  While the pre-fetch claimed it had worked, it only actually queried the ManyToManyField results for the first matching ContentType of the GenericForeignKey.  Looking at the code, this is a deliberate design decision, but it is not documented anywhere I could see.

I understand why this restriction is in place and so am not asking for a feature enhancement.  Instead, I think the documentation for this function needs to make clear that pre-fetching objects referenced by GenericForeignKeys is only supported if the query has restricted the key to one ContentType."	Cleanup/optimization	closed	Documentation	dev	Normal	fixed		loic@…	Ready for checkin	1	0	0	0	0	0