Should NHibernate assign id to entities or should it be handled by application?

Question

Hi

I'm writing an application and started to test my domain model entities. If I Create an instance of entity Company like this var company = new Company("my company"); I should get a valid entity, meaning the company should at this moment have an Id correct?

So the problem is that at the moment I have the Id generation made in the DB defined in an hbm file like this:

<id name="ObjectIdentity" column="CompanyId" type="System.Guid" unsaved-value="00000000-0000-0000-0000-000000000000">
  <generator class="guid.comb"/>
</id>

This causes a problem when writing unittests as I do not have en entity with an Id as it dosen't touch the db in the test, aka I have an invalid entity.

Now should I assign Id in the application and not let nhibernate be in charge of this or is this the wrong way to do it?

Answer 1

You need to save the entity to get an id if you're using db generated identity, if you wish to create your own identities - it is fine. do what suits you, just remember what you have chosen when you test.

I usually decide on each class which is more suitable for me - either DB generated or my own.

Answer 2

You need to call Session.Save on the entity before a guid is generated. You can call Session.Save to generate the entity without actually saving it to the database. This article does a pretty decent job of explaining it

Answer 3

Nhibernate should generate Id's for you. Id property of the entity must be protected. The best way is to use hilo

Answer 4

In most cases you should be letting NHibernate do its job which is to handle persistence. This is important since it allows you to easily change things (we went from identity to hilo mid-project).

I would question why you care that a newly created object has an id or not? From a business point of view a persistence ID is not relevant and shouldn't be checked with via unit tests. This as mentioned is the domain of integration tests. You should be careful in how you are using an objects persistence Id throughout the rest of your application. Remember this should NOT be treated as the objects business id/key.

Answer 5

Neither NHibernate nor the Application should be handling identifiers. Leave it to the database since this is the only concrete store of your data, and it is the only part of your application that knows what IDs have already been assigned and what ones are available.

Make an identity primary key column on your database table:

CREATE TABLE dbo.sample (
id int primary key identity(1,1),
...
...
...)

map your entities like this:

<id name="ID" column="id">
<generator class="identity" />
</id>

The Primary key will be generated automatically by the database when you save a new entity for the first time. IDENTITY(1,1) means "give new rows an Identity start at '1' then each subsequent row gets incremented by 1": so 1,2,3,4,5,6,7