map order-by exception

pirobox · **Joined:** Sun Jun 03, 2007 10:09 am **Posts:** 1

I've just started using NHibernate 1.2 and I've a problem using <map> tags.

I tried to write this in my hbm.xml file:
...
<map name="BookMarks" lazy="false" order-by="Id">
...

and I continue gettin the same exception:
Could not compile the mapping document: Pirobox.BusinessLayer.BookMark.PbBookMarkDirectory.hbm.xml ---> NHibernate.MappingException
: Cannot use order-by with generic map, no appropriate collection implementation is available

I have understood that the problem is with the the order-by clause, infact if I remove it everything works correctly. My question is (eventually), shouldn't NHibernate manage "order-by" for map tags? In the faqs I read that it should use internally the ListDictionary collection. Am I missing something?

rmswalsh · **Joined:** Fri May 25, 2007 5:55 am **Posts:** 26

Hi Pirobox,

I have seen this error too. Is it not supported in NHibernate. I have tried using the sort="natural" as well in the mapping and it complains about that too. Let me know how you got on.

karlchu · **Posted:** Thu Sep 13, 2007 9:59 am

A dictionary/hashtable is by definition unordered; one gets access to an element by specifying a key like this:

Code:

value = dictionary[key];

Therefore, specifying "order-by" for a <map> does not make much semantical sense. If the order of items is important, consider using a different mapping strategy like a <list>.

rmswalsh · **Joined:** Fri May 25, 2007 5:55 am **Posts:** 26

Fair point. There is a ListDictionary though.

In any case what you are saying is correct.