I have two test tables STUDY and EXPERIMENT

Experiments belong to Studies, so the EXPERIMENT table has a foreign key contraint STUDY_ID (primary key from STUDY table)

When creating a foreign key constraint in the mapping file,

<bag name="Experiments" cascade="all-delete-orphan">
<key column="STUDY_ID" not-null="true" foreign-key="FK_EXPERIMENT_STUDY"/> <!--Foreign Key in Experiment Table-->
<one-to-many class="Experiment"/>
</bag>


if I set the not-null="true" for the constraint, I get an error on saving the a study with experiments.

{"ORA-01400: cannot insert NULL into (\"MYDATABASE\".\"EXPERIMENT\".\"STUDY_ID\")"}

Basically, the foreign key STUDY_ID in the Experiment Table is correctly preventing NHibernate from inserting a null value into it.

It seems that NHibernate, is saving the child first 'Experiment'
then saving the parent 'Study' and then trying to update the foreign key STUDY_ID in the Experiment Table

If I remove the not-null="true" for the constraint, then it all works ... BUT I need it constrained.

my C# code is like this...
Study study = new Study();
study.Name = "Test Study";
study.Description = "Test Study Description";
Experiment experiment = new Experiment{ };
experiment.Name = "Test Experiment";
experiment.Description = "Test Experiment Description";
study.Experiments = new List<Experiment> {experiment};
session.Save(study);

Any Ideas would be appreciated.