An example on how cool and extensible Django is - add a completely new relational model field type in 80 lines of code:
My DenormField simply listens to a trio of the relevant signals, and then updates this value whenever it detects that the related User object has been updated, so it only costs on write (which, let's face it, is much better than on every read).
It is very common on busy databases to have part of a relation instanced (here is being called denormalized) in the same row that references it, in order to skip a JOIN on every single read. This extension provides a new model field type for the Django ORM that does all the required work for you. A single line of code in your model will allow it to automatically update the denormalized fields even when they are changed in the foreign row.