Horde RDO permits to define relationships over different db tables and 
makes access easy. With a ONE:MANY relationship between data types A 
and B, accessing a B from a given A is as easy as A->B[i].

In some cases it is desirable to have the returned multitude of B 
objects in a predetermined order. For instance, elements B might each 
have an element x and A->B could return the collection of elements B 
sorted by x in descending order:

A->B[0]->x > A->B[1]->x > A->B[2]->x ...

Ralf Lang suggested in the dev mailinglist  that this could be 
implemented through the mapper or in the Rdo_Query object. He details 
that eager relations are implemented as joins while lazy relations are 
essentially an automated way to implement the above. So the requested 
feature would require two implementations.

It seems to me that in both variants the sorting should be done on the 
DB side to avoid unnecessary overhead.