Why does NHibernate first insert and then update element of a collection?

Question

I'm not sure if it's a correct behavior or something done wrong on my side:

I've got a very simple parent-child relationship

public class SubmittedBatch
{
    public virtual Guid Id { get; set; }
    public virtual IList<SubmittedBatchParameter> Parameters { get; private set; }
}

public class SubmittedBatchParameter
{
    public virtual string Value { get; set; }
}

And with FluentNH it is configured like:

mapping.HasMany<SubmittedBatchParameter>(sb => sb.Parameters).Cascade.All();

I do something easy like adding a new element to the Parameters collection and then call SaveOrUpdate on the parent element.

Looking at the trace of the SQL Statements I get an insert:

INSERT INTO [SubmittedBatchParameter]
           (Value)
VALUES     ('Disabled' /* @p0 */)
select SCOPE_IDENTITY()

and then an update:

UPDATE [SubmittedBatchParameter]
SET    SubmittedBatch_id = '209971b7-c311-46bd-b989-9cf80113654c' /* @p0_0 */
WHERE  Id = 39 /* @p1_0 */

Why isn't NH just doing the Insert with also the Guid specified? Is this correct, or am I doing something wrong?

Thank you Simone

Answer 1

You have not mapped the reverse parent relationship explicitly. Therefore the only way that NHibernate knows to save the SubmittedBatch_id column value is when you save the parent object. It can't save both objects at once, so what it does is to save the child, then when it saves the parent, save the relationship.

In fact, even if you were to map both directions of the relationship, you would still have to specify which is the "master" by marking the other as an "inverse" relationship. The field is then updated by saving the master side.

So, if you were to map a SubmittedBatch property in the SubmittedBatchParameter class, map this as the "master" (i.e. set the collection mapping as the inverse using .Inverse in Fluent), and then set that when you add the parameter to the batch, then you would see just the insert that you expect.

Here's what I mean:

public class SubmittedBatch
{
    public virtual Guid Id { get; set; }
    public virtual IList<SubmittedBatchParameter> Parameters { get; private set; }
}

public class SubmittedBatchParameter
{
    public virtual SubmittedBatch SubmittedBatch { get; set; }
    public virtual string Value { get; set; }
}

Then in child mapping:

HasMany<SubmittedBatchParameter>(sb => sb.Parameters).Inverse();

and in parent mapping:

References(sbp => sbp.SubmittedBatch);