Question

I have WPF application with simple logic: we have Tasks and Projects entities. Something like this:

class Task
{
    public Project Prj; 
}

On application_start domain and session are created. And on first select query with prefetch (example of code at the end of message) I don't have any problem.

Then, I create new task, select project for it, and call

disconnectedState.ApplyChanges();

Everything is fine: I have new record in database, and I have

task.Prj == null // false

After that I call code with prefetch again. It selects all tasks (and already added one too). But: there is NO Prj in task, which I've already added.

task.Prj == null // true

And trying to access this field throws NullReferenceException.

There is Project.Id in database for this record. When I restart application (rebuild domain) everything works fine. It only happens with new added records.

Example of code:

using (disconnectedState.Connect())
{
    var tasksQuery = Library.Tasks.All()
        .Prefetch(z => z.Project)
        .Prefetch(z => z.User);
    foreach (var task in tasksQuery)
    {
        _tasks.Add(task);

    }
}

Help me, please. Maybe, I don't understand something, but I'm ready to read and learn this :)

Accepted Answer

I tried to reproduce this situation and review my code.

Sorry, there was my bug: one of the events sets Project = null in task object.

So, there is no bug in selecting entities with DisconnectedState.

But now I has another problem: I don't understand behavior of DO in this situation.


//Session is opened at application startup
disconnectedScope.DisposeSafely();
disconnectedState = new DisconnectedState();
disconnectedScope = disconnectedState.Attach(App.Session);

using (disconnectedState.Connect())
{
    var tasksQuery = Query.All<task>();//.Prefetch(z => z.Project);
    foreach (var task in tasksQuery)
    {
        //everything works fine
        var xx = task.Project.Name;

    }
}

Task tempTask;

using (var transactionScope = Transaction.Open())
{
    //Projects - collection of projects selected previously
    var task = new Task() { Project = Projects.FirstOrDefault()};
    task.Name = "xxx";

    tempTask = task;

    transactionScope.Complete();

}
disconnectedState.ApplyChanges();

//now there is record in db

//everything is ok
var pId = tempTask.Project.Id;

//I find similar situation in my code, and link to this object 
// EXISTED in memory, so, I add this line of code to reproduce situation.
// It is bug in my code, but now I don't uderstand such behavior :(
tempTask.Project = null;

//sync = true, but in fact, not (project is set to null, but in db - no)
// Maybe I don't understand behavior of
// PersistenceState
var sync = tempTask.PersistenceState == PersistenceState.Synchronized;

// trying to select all again from db
using (Transaction.Open(App.Session))
{
    using (disconnectedState.Connect())
    {
        var tasksQuery = Query.All<task>(); //.Prefetch(z => z.Project);
        foreach (var task in tasksQuery)
        {
            //NullReferenceException on previously added task "xxx"
            // this entity was'n selected from db, and has Project == null
            var xx = task.Project.Name;

        }
    }
}

Sorry if I missed explanation in the manual.

Answer 2

Initially, it looks weird, i.e. probably, that's a result of some bug.

Few questions to help us to reproduce it:

Are Task.Project and Project.Task properties paired by some way?
In your example, Prj property isn't marked as [Field] - but actually it is?
Does removal of .Prefetch change anything? Try to replace .Prefetch with for-loop that simply accesses Project and User properties and see what happens.

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