WPF MVVM-DataGrid不会将更改更新回数据库

Question

I have a WPF DataGrid that is bound to a ObservableCollection that is in my NinjaList ViewModel

public ObservableCollection<NinjaVM> Ninjas { get; set; }

And the method where Ninjas is defined

public NinjaListVM()
        {
            using (var context = new NinjaApp_DatabaseEntities())
            {
                var ninjas = context.ninjas.ToList();
                Ninjas = new ObservableCollection<NinjaVM>(ninjas.Select(r => new NinjaVM(r)));
            }
        }

The code in my View is as followed

<Window 
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:NinjaApp_V2.Views"
        xmlns:ViewModel="clr-namespace:NinjaApp_V2.ViewModel" xmlns:NinjaApp_V2="clr-namespace:NinjaApp_V2" x:Name="NinjaCRUDWindow" x:Class="NinjaApp_V2.Views.NinjaCRUD"
        mc:Ignorable="d"
        Title="NinjaCRUD" Height="300" Width="300"
        DataContext="{Binding Ninjas, Source={StaticResource Locator}}" Loaded="onLoad">


    <Grid Margin="0,10,3.6,0.4">
        <DataGrid x:Name="DataGridNinjas" ItemsSource="{Binding Ninjas, Mode=TwoWay}" SelectedValue="{Binding Ninjas, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" AutoGenerateColumns="True">
        </DataGrid>
        <Button x:Name="btnNewNinja" Command="{Binding ShowNewNinja}"  Margin="10,228,103.4,-0.4" Content="New Ninja"/>
    </Grid>
</Window>

The view does show all the contents of the Ninja table into the Grid. Also when changes are made they persist throughout the time that the application is running. But changes are not being saved to the database. My understanding is that if you bind to a ObservableCollection like this using Mode = TwoWay that it should automatically update the changes back to the database. But clearly I am missing something, can somebody perhaps point out what I am doing wrong?

I am using MvvM Light as MvvM framework.

Answer 1

Yeah, the reason is that your Viewmodels haven't a direct access to the Database.

        //opening the database
        using (var context = new NinjaApp_DatabaseEntities()) 
        {
           //query the database and store the result in memory
            var ninjas = context.ninjas.ToList();

           //Viewmodel mapping
            Ninjas = new ObservableCollection<NinjaVM>(ninjas.Select(r => new NinjaVM(r)));
        } // in the end of the using statement the database will be "closed"

Manipulating an ninja doesn't affect the database.

I suspect you are using the entity framework.

so if you want to store the changes, you must reopen the Database, search for the specific ninja and overwrite the property and use the SaveChanges Methode.

For practicing you can do this in the setter

    public class NinjaVM
{

    private int _id;

    private string _name;
    public string Name
    {
        get { return _name; }
        set
        { 
            using (var context = new NinjaApp_DatabaseEntities())
            {
                var ninja = context.ninjas.FirstOrDefault(n => n.Id == _id  ));
                if(ninja == null)
                return;
                ninja.Name = value;
                context.SaveChanges();
            }
        }
    }