In doing some experimentation and research to resolve a problem, I came across the following comment from Sergey (late last year):

[url=http://forum.hibernate.org/viewtopic.php?t=967618&highlight=ilist+index+key]
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First, can someone confirm that this is correct??

I love NHibernate and have been using it now for some time. I have spent many hours reading documentation, blogs, examples, etc. All of which refers to <list>, <bag> and <map> being the various collection types available.

I had read that <list> and <map> do not work with bi-directional associations but nowhere (until today!) had I read that <list> and <map> simply do not work at all with <one-to-many> collections.

If this is true, it seems misleading that this major shortcoming of NHibernate is not mentioned in the documentation.

I don't know where you found my comment, but yes, I must have meant bi-directional (i.e. inverse="true") one-to-many associations.

Hi Sergey,

Thanks for clearing up.

(BTW found it in post # 967618).

However, a simple <list> or <map> (without inverse="true"), appears to generate an INSERT and an UPDATE for each item added to the list and therefore not really workable in production app.

Or am I missing something?

Thanks,
Phil

That happens because NHibernate maintains the association separately from the object itself. Hibernate already allows using inverse with indexed collections, so some future version of NHibernate will allow it too, but I don't want to speculate on when it will happen.

I am sure the bi-directional capability will be very welcome whenever that feature is made available.

However, does that mean there are no plans to correct the basic problem of a uni-directional <list> or <map> (i.e. without an inverse="true")? Surely the unnecessary calls seriously undermine the possibility of using NHibernate real-world enterprise-capable applications?

Is there any scenario where you would use a <list> or <map> in an app built using NHibernate?

After trying <list> and <map> and failing, we resorted to <bag> for every one-to-many collection in our application, which has almost 400 entities. All are marked inverse="true" and most have a sort-order attribute defined.

Having to use <bag> rather than <list> or <map> was one of the most frustrating and confusing parts of learning NHibernate ...

Glad to hear I'm not alone.

I guess the part which compounds the frustration is that the documentation for Hibernate and NHibernate both talk about <list> and <map> as if they are choices on par with <bag> and <set>.

However, from what I can surmise, it appears that:
- one-to-many collections only work properly (i.e. one sql call per row insert) if they are bi-directional, and
- <list> and <map> cannot be bi-directional.

There should be a big bold statement at the top of the NHibernate documentation connecting the dots on the above two statements and warning newcomers looking to use NHibernate in a production app : DO NOT USE <LIST> OR <MAP> FOR ONE-TO-MANY COLLECTIONS.