nHibernate: determine property value before save

Question

we are currently evaluating whether nHibernate supports the requirements for our project. We share the database with another application so that we are not completely free as regards changes to the schema. Some columns are filled with unique and consecutive numbers (e.g. for invoices). The next number is determined by a stored procedure that also implements a locking algorithm so that the numbers are guaranteed to be consecutive. On the one hand we could define a trigger on the respective tables that sets the value for the column when an empty or special value is provided. This would require changing the existing database definition - though it might be the most reliable way to implement this. In order to avoid the change of the database definition we are trying to solve this in the nHibernate ORM. We've first tried to implement a user type that calls the stored procedure in NullSafeSet if an empty value is provided. Unfortunately, the connection and transaction of the provided command are not set yet when NullSafeSet is called.

How can we solve this with nHibernate?

Thanks in advance,

Markus

Answer 1

If you decide to go with trigger route, then you'll need to add generated attribute to your property mapping.

Generated properties are properties which have their values generated by the database. Typically, NHibernate applications needed to Refresh objects which contain any properties for which the database was generating values. Marking properties as generated, however, lets the application delegate this responsibility to NHibernate. Essentially, whenever NHibernate issues an SQL INSERT or UPDATE for an entity which has defined generated properties, it immediately issues a select afterwards to retrieve the generated values.

Aside from that, I'm not quite sure how would you call stored procedure from NHibernate issued INSERT, without adding a trigger or default constraint on column.

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Edit

Looks like NHibernate has a notion of class persisters, through the interface IEntityPersister. Maybe you could hack something out from that.

The persister attribute lets you customize the persistence strategy used for the class. You may, for example, specify your own subclass of NHibernate.Persister.EntityPersister or you might even provide a completely new implementation of the interface NHibernate.Persister.IClassPersister that implements persistence via, for example, stored procedure calls, serialization to flat files or LDAP. See NHibernate.DomainModel.CustomPersister for a simple example (of "persistence" to a Hashtable).

You could start from NHibernate's source.

If you have the ability to add triggers to database, that would probably be the best, straightforward way, without investing too much time to fight with NHibernate's internals.