NHibernate ISession.Save() - Why is this persisting my entities immediately? - nhibernate

Question

I am creating a large number of entities with NHibernate, attaching them to my ISession, and then using a transaction to commit my changes to the database. Code sample is below:
ISession _context = SessionProvider.OpenSession();
//Create new entities
for(int i=0; i<100; i++)
{
MyEntity entity = new MyEntity(i);
//Attach new entity to the context
_context.Save(entity);
}
//Persist all changes to the database
using(var tx = _context.BeginTransaction())
{
//Flush the session
tx.Commit();
}
I was under the impression that the line _context.Save() simply makes the ISession aware of the new entity, but that no changes are persisted to the database until I Flush the session via the line tx.Commit().
What I've observed though, is that the database gets a new entity every time I call _context.Save(). I end up with too many individual calls to the database as a result.
Does anyone know why ISession.Save() is automatically persisting changes? Have I misunderstood something about how NHibernate behaves? Thanks.
***EDIT - Just to clarify (in light of the two suggested answers) - my problem here is that the database IS getting updated as soon as I call _context.Save(). I don't expect this to happen. I expect nothing to be inserted into the database until I call tx.Commit(). Neither of the two suggested answers so far helps with this unfortunately.
Some good information on identity generators can be found here

Answer 1

Try:
using(Session _context = SessionProvider.OpenSession())
using(var tx = _context.BeginTransaction())
{
//Create new entities
for(int i=0; i<100; i++)
{
MyEntity entity = new MyEntity(i);
//Attach new entity to the context
_context.Save(entity);
}
//Flush the session
tx.Commit();
}

Answer 2

Which identity generator are you using? If you are using post-insert generators like MSSQL/MySQL's Identity or Oracle's sequence to generate the value of your Id fields, that is your problem.
From NHibernate POID Generators Revealed:
Post insert generators, as the name
suggest, assigns the id’s after the
entity is stored in the database. A
select statement is executed against
database. They have many drawbacks,
and in my opinion they must be used
only on brownfield projects. Those
generators are what WE DO NOT SUGGEST
as NH Team.
Some of the drawbacks are the
following
Unit Of Work is broken with the use of
those strategies. It doesn’t matter if
you’re using FlushMode.Commit, each
Save results in an insert statement
against DB. As a best practice, we
should defer insertions to the commit,
but using a post insert generator
makes it commit on save (which is what
UoW doesn’t do).
Those strategies
nullify batcher, you can’t take the
advantage of sending multiple queries
at once(as it must go to database at
the time of Save)

Answer 3

You can set your batch size in your configuration:
<add key="hibernate.batch_size" value="10" />
Or you can set it in code. And make sure you do your saves within a transaction scope.

Answer 4

Try setting the FlushMode to Commit:
ISession _context = SessionProvider.OpenSession();
context.FlushMode = FlushMode.Commit;
peer's suggestion to set the batch size is good also.
My understanding is that when using database identity columns, NHibernate will defer inserts until the session is flushed unless it needs to perform the insert in order to retrieve a foreign key or ensure that a query returns the expected results.

Answer 5

Well
rebelliard's answer is a possibility depending on your mapping
you are not using explicit transactions (StuffHappens' answer)
default flush mode is auto and that complicates things (Jamie Ide's answer)
if by any change you make any queries using the nhibernate api the default behaviour is to flush the cache to the database first so that the results of those queries will match the session entity representation.

Answer 6

What about :
ISession _context = SessionProvider.OpenSession();
//Persist all changes to the database
using(var tx = _context.BeginTransaction())
{
//Create new entities
for(int i=0; i<100; i++)
{
MyEntity entity = new MyEntity(i);
//Attach new entity to the context
_context.Save(entity);
}
//Flush the session
tx.Commit();
}