Updating a foreign key to Null value using nhibernate - asp.net

Question

I have two table BuildGroup and table DocumentTemplate. DocumentTemplate table has BuildGroupId as foreign key which is nullable. In a certain senario I update BuildGroupId in DocumentTemplate table.
public bool EditDocTempForBldGrp(int docId, int bldGrpId)
{
try
{
using (ISession session = Document.OpenSession())
{
using (ITransaction transaction = session.BeginTransaction())
{
HSDocumentTemplate objDocBO = new HSDocumentTemplate();
objDocBO = GetDocumentDetailsById(docId);
HSBuildGroup objBldGrp = new HSBuildGroup();
if (bldGrpId != 0)
{
objBldGrp.Id = bldGrpId;
}
else
{
//int ? bldid = null;
//objDocBO.HSBuildGroup.Id = null;
//objDocBO.HSBuildGroup.Id = DBNull.Value;
//objDocBO.HSBuildGroup.Id = -1;
}
objDocBO.HSBuildGroup = objBldGrp;
session.Update(objDocBO);
transaction.Commit();
}
}
return true;
}
catch (Exception ex)
{
Console.WriteLine(ex.Message);
return false;
}
}
In another senario I need to set BuildGroupId in DocumentTemplate table again to
dbnull.value. I tried with different cases as in else block. It giving the error : Cannot
implicitly convert type 'System.DBNull' to 'int'.
How can I update a foreign key value with NULL? Any help will be appreciated.

Answer 1

A few observations:
You should not assign the Id to a HSBuildGroup object, but instead load the instance via Session.Load() in case the bldGrpId is not 0.
You can just set the build group of a document to null like this:
objDocBO.HSBuildGroup = null;
NHibernate will take care of the rest.
Hope that helps.

Answer 2

Use Session.Load and null as Kay Herzam said.
You must to be sure than the document exist to call session.Load. Example:
public bool EditDocTempForBldGrp(int docId, int bldGrpId)
{
try
{
using (ISession session = Document.OpenSession())
using (ITransaction transaction = session.BeginTransaction())
{
session.Get<HSDocumentTemplate>(docId).HSBuildGroup
= bldGrpId = 0 ? null : session.Load<HSBuildGroup>(bldGrpId);
transaction.Commit();
}
}
}
This way nhibernate will execute something like,
select ... from DocumentTemplate where DocId = ..;
UPDATE DocumentTemplate SET .... , BuildGroupId = null where DocumentId = XX;
or
select ... from DocumentTemplate where DocId = ..;
UPDATE DocumentTemplate SET .... , BuildGroupId = YY where DocumentId = XX;
Note there is no select for BuildGroup to the database.
If you are not sure about the existence of the build group, your code should look like:
public bool EditDocTempForBldGrp(int docId, int bldGrpId)
{
try
{
using (ISession session = Document.OpenSession())
using (ITransaction transaction = session.BeginTransaction())
{
session.Get<HSDocumentTemplate>(docId).HSBuildGroup
= session.Get<HSBuildGroup>(bldGrpId);
transaction.Commit();
}
}
}
This way nhibernate will execute something like,
select ... from DocumentTemplate where DocId = ..;
SELECT .... FROM BuildGroup where buildgroupid = ZZ;
UPDATE DocumentTemplate SET .... , BuildGroupId = null where DocumentId = XX;
Get automatically returns null if the object doesn't exist.
Finally you don't need to call Session.Update() this is for reattaching an entity. Everything associated to a Session will be flushed when you commit the transaction.