NHibernate一直在要求LAST_INSERT_ID吗？

Question

I have a simple object:

class Entity {
    public virtual int Id { get; set; }
    public virtual string Text { get; set; }
}

If I insert 10 entities using the same transaction, I get the following calls:

INSERT INTO ...
SELECT LAST_INSERT_ID()

INSERT INTO ...
SELECT LAST_INSERT_ID()

...

INSERT INTO ...
SELECT LAST_INSERT_ID()

I'm not using those entities anymore, just inserting them.

I saw that while using the logger to see the SQL generated when writing some complex query, and then I noticed it happens with the other tests too.

Is that the normal behavior?

Answer 1

This is normal if you have the primary key numbering strategy set to autoincrement. If you use a different strategy, like low-high or another algorithmic mechanism, it won't be necessary, because NHibernate can set the ID itself. There's a blog post on primary key strategies, which may not be up to date on all the available Id generation mechanisms, but has the main ones covered. There's some performance and related information about the different strategies you can choose here . If memory serves me, using a sequence generator may be more performant on databases that support them, because Nhibernate can (at least theoretically) ask for them before using them.

For the mapping of your Id property to work as you'd expect, finding out what Id was just inserted is important. Also, for mapping any associations that depend on that primary key, the ORM needs to know the primary key of the root object, so you'll likely see a round of updates on any inserted association records. (This is also the reason why one side of the association should set the Inverse property).

Answer 2

This is normal behavior if your key fields in your model mapping define the Generators.Identity generator.

If you inspect the ID field of the model you just persisted, it will now contain the actual database value.