FNhibernate, GeneratedBy.HiLo, hibernate_unique_key etc.

Question

Hi,

I have started using the s#arp architecture which uses FNhibernate and GeneratedBy.HiLo to generate primary keys (there is also table hibernate_unique_key). Apparently, this is recommended practise and I would like to stick with this. Now to my problem. I have used NHibernate and hbm mapping quite a bit and usually used identity columns for my primary keys. This allowed me to seed the database using SQL. Can I do this with the aforementioned setup (hibernate_unique_key table etc.). I need to do this as SQL insert is much more efficient than using NHibernate + C# to seed the db with a million entities. Any feedback would be very much appreciated. Thanks.

Christian

Answer 1

I guess the problem is that the pk generation is controlled by nhibernate and not the db. so an option would be to use instance.GeneratedBy.Identity(). do you reckon that would be sensible?

I would really appreciate any comments.

Christian

Answer 2

You should be able to seed the database using plain SQL and still use HiLo to generate the primary keys in NHibernate. What you have to do is to set the NextHi value(s) in the HiLo table to values that are high enough that the next entity you save will get an id that is higher than the highest id set when you seed the database.

So, you should be able to do something like this:

• run the schema export
• seed the database using a custom sql script (you would have to supply your own id's in the script, since they are not generated by the database)
• manually insert a big enough value into the hibernate_unique_key table, so that the next id generated by NHibernate is larger than the largest inserted in the seeding
• use NHibernate as usual

There are a few different approaches to using HiLo with NHibernate (one shared next-hi for all entities, a next hi per entity, etc.) so you might have to do a little experimenting to find out what value(s) would be appropriate to write to the hibernate_unique_key table after the seeding, depending on your hilo strategy and what max_lo you are using etc.

As a side note, schema export does not seem to support multiple rows in the hibernate_unique_key table that well, so you might have to do some manual stuff to create all the rows in the table if you use a hilo row per entity.

You could also use Identity to generate the ids, but at the cost of worse performance with NHibernate. The reason for the performance loss is that NHibernate has to do an extra read for each insert to get the id that was generated by the database. With hilo NHibernate already knows the id that the entity will get, so there is no need for that extra read.

Another option could be to use GuidComb, which also allows NHibernate to generate the ids, and therefore removes the need to query the database to get the id after an insert. However, you then have to look at ugly guids instead of nice integers when developing. :)

Answer 3

Hello Christian,

Maybe it's a bit late but the Identity generator will break the UnitOfWork-pattern.

If you perform a Save on your currentSession it will already try to insert the entity in the DB and thus break the whole meaning of the UoW. After many hours I found the reason why it was broken and the reason was of this Identity Generator. I use now the HiLo generator.

Following links helped me through this:

Nice article about the behaviour of these generators

Regards