Fluent NHibernate Many to many not saving

Question

I'm having a problem trying to get a many to many relationship to save the join record in NHibernate using Automapper in Fluent NHibernate.

There are quite a few other posts about this on S.O but so far none of them have solved the problem, wondering if I'm doing something different some how.

I have the app setup to map, and when i manually create the join record in the DB i do get back the data, so on a read level it is mapping correctly, but it won't persist the relationship.

Here's the relevant mapping. I was using automapper with default conventions but eventually tried this based on another SO post.

.Mappings(m => {
                        m.AutoMappings.Add(AutoMap.AssemblyOf<User>);
                        m.AutoMappings.Add(AutoMap.AssemblyOf<PostalCode>);
                        m.AutoMappings.Add(
                                AutoMap.AssemblyOf<VPA>()
                                    .Override<VPA>(v => 
                                        v.HasManyToMany(x => x.PostalCodes)
                                            .Table("PostalCodesToVPAs")
                                            .ParentKeyColumn("PostalCode_Id")
                                            .ChildKeyColumn("VPA_Id")
                                            .Cascade.SaveUpdate())
                                    .Override<PostalCode>(p => 
                                        p.HasManyToMany(x => x.VPAs)
                                            .Table("PostalCodesToVPAs")
                                            .ParentKeyColumn("VPA_Id")
                                            .ChildKeyColumn("PostalCode_Id")
                                            .Cascade.SaveUpdate().Inverse())
                            );
                    })

My actually save looks like this. It may be overkill as i'm explicitly saving both postal code and vpa but i read a lot about people having problems with the inverse line in the mapping so i wanted to try both. It didn't work.

var postalCode = new PostalCode {Value ="90210", CreatedBy = 0, CreationDate = DateTime.Now, ModifiedBy = 0, ModifiedDate = DateTime.Now};
vpa.PostalCodes.Add(postalCode);
postalCode.VPAs.Add(vpa);
PostalCodeService.Save(postalCode);
VPAService.Save(vpa);

under the covers the service.save calls do

return (int)Session.Save(obj);

Has anyone seen this and know why it wouldn't save the join record?

Answer 1

Figured out the solution, wanted to update in case anyone runs into the same problem.

You have to wrap the save call in a transaction or it doesn't commit the join record. I ended up modifying that service code to read

using (var tx = Session.BeginTransaction()) {
    Session.Save(obj)
    tx.commit();
    return obj.Id;
}