Hi List,
I'm responsible for looking at NHibernate as a potential DB solution
on our next product release -- but I'm having trouble getting my
prototype to work properly. In a few cases, I'm getting perplexing
behaviour associated with lazy loading and proxies. I suspect that
these are all occuring because I have some error in my understanding
of how these work.
Here's my first problem. I'm encountering a
LazyInitializationException when my WebApp testing front end tries to
display a DataGrid with information about objects of type Instrument.
---------------- Application information ------------------
The application manages electronic medical instruments in a hospital.
Part of our object system is a heiarchy of parent/child relationships
(parent owns a collection of children; children have a reference to
their parent; I don't think either end is set inverse="true")
Institution (e.g. Springfield Hospital)
Location (e.g. Emergency Room)
Instrument
Institution and Location are effectively "virtual" classes (though
they're not explicitly marked as such); they each have 2 subclasses
that are used in the database.
References in the source code to other objects such as
HealthCareSystem and Method are unimportant.
------------------------------------ Source Code
-------------------------------------------
<Locations.aspx.cs>
...
private void BindInstrumentGrid()
{
...
grInstr.DataSource =
Facade.InstLoc.GetInstruments(<various parameters>);
grInstr.DataBind();
}
<InstrumentFacade.cs>
...
public IList GetInstruments(<various parameters>)
{
IList<Instrument> instrumentList;
...
using (ISession ses = NHibernateHelper.OpenSession())
using (ses.BeginTransaction())
{
instrumentList = ses.CreateQuery(
@"SELECT instr FROM Instrument AS instr
JOIN instr.Method AS md
JOIN md.Method AS meth
JOIN instr.Location AS loc
JOIN loc.Institution AS instit
JOIN instit.HCSystem AS hcs
WHERE meth.Name=:method_name
AND loc.Name=:location_name
AND instit.Name=:institution_name
AND hcs.Name=:healthcaresystem_name")
.SetString("method_name", methodName)
.SetString("location_name", asLocationName)
.SetString("institution_name", asInstName)
.SetString("healthcaresystem_name",
asHCSystemName)
.List<Instrument>();
foreach (Instrument i in instrumentList)
{
NHibernateUtil.Initialize(i.Location);
NHibernateUtil.Initialize(i.Location.Institution);
}
ses.Transaction.Commit();
}
return instrumentList.ToList();
}
...
----------------------------------------------------------------------------------------------------
On application execution, when BindInstrumentGrid() executes
"grInstr.DataBind();" I get the following NHibernate exception:
"Initializing[TestRecordProto.Domain.Institution#2]-Could not
initialize proxy - no Session."
I partially understand why I'm getting this error, because I'm seeing
weird behavior during NHibernateUtil.Initialize() when I run in debug
mode. Initially, i.Location is of type LocationProxy -- with its only
set property being "string Name". All other properties, including the
int ID, is uninitialized. But after "Initialize(i.Location)", I would
expect i.Location proxy would be replaced with a real object of type
Location. But it is not in fact replaced -- the reference is left as
a proxy, and its properties do not change! And of course then
"Initialize(i.Location.Institution)" can not really initialize the
Institution field -- because i.Location.Institution is null, because
i.Location is a proxy without instantiated properties!
So I don't know what I'm doing wrong -- I'm supposed to call
Initialize() on the referenced objects that I'll need to access after
the session is closed -- and yet Initialize() isn't solving the
problem for me. Why does this happen?
The information in the database all looks correct, with the correct
links between these objects. I have 1 record saved of type
Instrument, which is linked by foreign key to a valid Location, which
is linked by foreign key to a valid Institution.
Thank you in advance.
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