EOSortOrdering based on a @count relationship

Hi,
I've been trying to isolate the issue with this, but I just can't  
even get an error...
So I need to sort based on a calculated attribute: relationship 
().count()

but I just can't make it work...

I've got entity "a" which has a toMany relationship "relationship()".
I need to sort "a" entities based on relationship().count().

So here's my SortOrder:
--------
     public Number numberOfComments() {
     	return (Number)writeCommentaries().count();
     }

	public NSArray sortOrderingByCommentaries() {
		NSMutableArray sortOrdering = new NSMutableArray();
		EOSortOrdering newSO = EOSortOrdering.sortOrderingWithKey 
("numberOfComments", EOSortOrdering.CompareDescending);
		sortOrdering.addObject(newSO);
		return (NSArray)sortOrdering;
	}
--------

and the actual sorted array:
--------
	public NSArray listOfNewsSortedByCommentaries() {
		return (NSArray)EOSortOrdering.sortedArrayUsingKeyOrderArray 
(listOfNews(), new NSArray(sortOrderingByCommentaries()));
	}
--------

but it won't work, I don't even get a crash, just a "no instance  
available"...
I tried with another Number attribute to see if it coud be the cause,  
but it works. I also sort using another attribute and display the  
relationship().count() for each "a" and it works. It seems I just  
can't sort using the relationship().count().

But I though I could use custom attribute as long as the array is  
already in an editing context.

I must miss something, but I can't find it...

Thanks for your help

Xavier