prefetch_related does not document restriction of requiring homogeneous results

[pab@…]

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.

[loic84]

I can confirm that the implementation relies on the assumption that all the retrieved objects are homogenous.

This limitation should be documented.

[Tim Graham]

​PR

[Tim Graham <timograham@…>]

In 69239c0f:

Fixed #21422 -- Documented that prefetch_related() results must be homogeneous.

[Tim Graham <timograham@…>]

In 683b3df:

[1.8.x] Fixed #21422 -- Documented that prefetch_related() results must be homogeneous.

Backport of 69239c0f968ee55cb9ca1e31323fb696db0c2aec from master

[Tim Graham <timograham@…>]

In 31a51a04:

[1.9.x] Fixed #21422 -- Documented that prefetch_related() results must be homogeneous.

Backport of 69239c0f968ee55cb9ca1e31323fb696db0c2aec from master