One of my usages of NHibernate is to move objects between two different databases; for this reason I'd set the id type to be assigned (since I never want an id generated if the object doesn't exist at destination).

However, there are some cases when I do need to create new entities and I don't want to manually do the job that auto_increment does, so it would make more sense to set the id types to 'generated' for this case, but I very rarely need to use the 'generated' behaviour. I don't really want to duplicate the entire mapping file and have to maintain two copies.

The code below copies an object from database A to database B, and preserves the id x whether the object exists in B or not. If I change the id types to generated, will this remain the case?

As a followup to my own question, what I really want is an assigned ID that behaves like a native ID when nothing's been assigned (or vice versa). Looks like other people are also interested in this (http://opensource.atlassian.com/projects/hibernate/browse/HBX-1034) - I guess a custom id generator is what's required? I can't find a lot about writing and using custom generators anywhere.

Thanks,

Steve

Or use

in your copier to save with an assigned ID, overriding the generator.

This won't work with child entities and so on; it would require me not to cascade anything I think.

I am not sure if this will work.

Write your own generator class. The generator will check for any assigned id. If an id exists, it returns the existing id. Otherwise it calls one of the NH supplied generator of your choice.

Your custom generator should inherit from NHibernate.Id.IIdentifierGenerator. You will need to link to the generator in the mapping files like the following

Yeah, that'll probably be the best solution. I'm not sure if it's that easy though, since looking at the GeneratorFactory code in the source there's some setup that goes on. I'll investigate sometime and maybe write it up somewhere, since there's very little on the web about it.