Why isn't the TreeView showing the Children - wpf

Question

I am writing a Program in .Net core (C#), which access a SQLite database using EF core.
namespace BtmManager.Models
{
public class Projekt
{
[Key]
public int ProjektId { set; get; }
[Required]
public int BtmBestandsbuchNr { get; set; }
[Required]
public int Stufenanzahl { get; set; }
public string Produktbezeichnung { get; set; }
[Required]
public int ProduktNr { get; set; }
public DateTime Zeitraum { get; set; }
public IList<Stufe> Stufen { get; set; }
}
public class Stufe
{
[Key]
public int StufId { set; get; }
[Required]
public int StufenNummer { get; set; }
public string MaterialName { get; set; }
[Required]
public int AnzahlEinträge { get; set; }
public int ProjektId { get; set; }
public Projekt Projekte { get; set; }
public IList<Eintrag> Einträge { get; set; }
}
public class Eintrag
{
[Key]
public int EintragId { set; get; }
[Required]
public byte Einheit { get; set; }
public string LfdNr { get; set; }
public DateTime Datum { get; set; }
public float Anfangsbestand { get; set; }
public float TheroZugang { get; set; }
public float PrakZugang { get; set; }
public float Arbeitsverlust { get; set; }
public float Abgang { get; set; }
public float AktuellerBestand { get; set; }
public string Bemerkung { get; set; }
public int StufId { get; set; }
public Stufe Stufen { get; set; }
}
}
I entered some testvalues
ProjektId BtmBestandsbuchNr Stufenanzahl Produktbezeichnung
1 jnjnijni 2 khih
2 phgztf 1 wdfw
3 yawesxdr 2 ewg
StufId StufenNummer MaterialName AnzahlEinträge ProjektId
1 1 nvjgh 3 1
2 2 jljk 3 1
The hierarchy is:
Projekt (Project)
Stufen (stage)
Eintrag (entry)
My goal is now to show this hierarchy in WPF in a TreeView.
<TreeView x:Name="TreeView" Margin="5,0,5,0" Grid.Row="1" Grid.RowSpan="7" ItemsSource="{Binding}">
<TreeView.Resources>
<HierarchicalDataTemplate DataType="{x:Type self:Projekt}" ItemsSource="{Binding Stufen}">
<TextBlock Text="{Binding Path=Produktbezeichnung}" />
</HierarchicalDataTemplate>
<DataTemplate DataType="{x:Type self:Stufe}">
<TextBlock Text="{Binding Path=MaterialName}" />
</DataTemplate>
</TreeView.Resources>
</TreeView>
As you can see, the Projects where displayed properly, but no children !
Any idea ??
In case you need to see the whole code, I uploaded it to GitHub

Answer 1

Looking at the source code.
The problem is caused by the default behaviour of entity framework.
Related entities are lazy ( rather than eager ) loaded by default.
If you select an entity, it's related children are not read.
You need to include related children - in this case your Stufen.
There are two ways to do this
How to include() nested child entity in linq
If i follow your entity names correctly:
.Include("Stufen").ToList();