NHibernate map by code with OneToMany association: Select is inefficient and Inserts fail (NHibernate.StaleStateException) - nhibernate

Question

I have a parent class and a child class. One Child is always related to just one parent, but a parent can have multiple children:
public class Parent
{
public virtual string Id { get; set; }
public virtual string Name { get; set; }
public virtual IList<Child> Children { get; set; } = new List<Child>();
}
public class Child
{
public virtual int Id { get; set; }
public virtual string ParentId { get; set; }
public virtual string Name { get; set; }
}
I'm using the latest version of NHibernate 5.1.3 and mapping by code:
internal class ParentMapping : ClassMapping<Parent>
{
public ParentMapping()
{
Table("Parent");
Id(x => x.Id);
Property(x => x.Name);
Bag(
x => x.Children,
map =>
{
map.Key(km => km.Column("ParentId"));
map.Lazy(CollectionLazy.NoLazy);
map.Cascade(Cascade.Persist);
map.Inverse(true);
},
x => x.OneToMany());
}
}
internal class ChildMapping : ClassMapping<Child>
{
public ChildMapping()
{
Table("Child");
Id(x => x.Id, x => x.Generator(Generators.Identity));
Property(x => x.ParentId);
Property(x => x.Name);
}
}
Queries work but are quite inefficient. Instead of creating a single JOIN statement to query the children together with their parent, an explicit SELECT is made to retrieve the Child objects.
Even worse, inserts result in the following error:
NHibernate.StaleStateException: 'Batch update returned unexpected row count from update; actual row count: 0; expected: 3'
Here's a sample of a query:
using (var session = _sessionProvider.GetSession())
return session.Query<T>().ToList();
And that's the code to save a new item:
using (var session = _sessionProvider.GetSession())
{
session.Transaction.Begin();
session.Save(newEntity);
session.Transaction.Commit();
}
So everything's pretty easy.
I assume, the Bag() configuration in ParentMapping needs to be fixed. What am I doing wrong?

Answer 1

Change the fetch strategy of the bag mapping to join will generate a join query, like this:
Bag(
e => e.Children,
map => {
map.Key(km => km.Column("ParentId"));
// map.Lazy(CollectionLazy.NoLazy);
// change fetch str
map.Fetch(CollectionFetchMode.Join);
map.Cascade(Cascade.Persist);
map.Inverse(true);
},
x => x.OneToMany()
);
And you have the collection persist as inverse (map.Inverse(true);), you should have a many to one mapping for Parent in your Child class like this:
public class Child {
public virtual int Id { get; set; }
// public virtual string ParentId { get; set; }
public virtual Parent Parent { get; set; }
public virtual string Name { get; set; }
}
Then map the Parent property as ManyToOne like this:
public ChildMapping() {
// other mapping goes here
ManyToOne(
x => x.Parent,
map => {
map.Column("ParentId");
map.Class(typeof(Parent));
map.Fetch(FetchKind.Join);
}
);
}
But nhibernate do not include children of parent by default (maybe it is too heavy), and if you want to query children with parent instance, you can query like this:
using (var session = OpenSession()) {
var query = session.Query<Parent>().Select(p => new {
Parent = p, Children = p.Children
});
var data = query.ToList();
}
For saving entities to database, should do like this:
try {
// save parent first
var parent = new Parent();
parent.Name = "Parent object";
session.Save(parent);
// then save child
var child = new Child();
child.Name = "Child object";
child.Parent = parent;
session.Save(child);
session.Flush();
tx.Commit();
}
catch (Exception) {
tx.Rollback();
throw;
}