WPF MVVM如何将属性绑定到更改上下文查询的comboBox？

Question

I have an WPF view with one Combobox and one DataGrid. I use Entity Framework database first context as a log-term context in my app. Let's say, this context wold be Global.DbContext . My entites created by EF are: Log and Client . In my XAML i have such bindigs:

<DataGrid ItemsSource = {Binding LogEntries} />
<ComboBox ItemsSource="{Binding Clients}" SelectedItem = {Binding SelectedClient} DisplayMemberPath="fullDomainName"
            IsSynchronizedWithCurrentItem="True"/>

In my view model i have these properties (i use Catel Framework, so the properties look a bit strange):

public ObservableCollection<Log> LogEntries
    {
        get { return GetValue<ObservableCollection<Log>>(LogEntriesProperty); }
        set { SetValue(LogEntriesProperty, value); }
    }

    public static readonly PropertyData LogEntriesProperty = RegisterProperty("LogEntries", typeof(ObservableCollection<Log>), null);

 public ObservableCollection<Client> Clients
    {
        get { return GetValue<ObservableCollection<Client>>(ClientsProperty); }
        set { SetValue(ClientsProperty, value); }
    }

    public static readonly PropertyData ClientsProperty = RegisterProperty("Clients", typeof(ObservableCollection<Client>), null);



    public Client SelectedClient
    {
        get { return GetValue<Client>(SelectedClientProperty); }
        set { SetValue(SelectedClientProperty, value); }
    }

    public static readonly PropertyData SelectedClientProperty = RegisterProperty("SelectedClient", typeof(Client), null);

and a constructor:

public LogWindowViewModel()
    {


        Global.DbContext.Clients.Load();
        Clients = Global.DbContext.Clients.Local;

        var qry = Global.DbContext.Logs.Where(c => c.client_id == SelectedClient.client_id);
        qry.Load();
        LogEntries = new ObservableCollection<Log>(qry);
    }

which is not working, because at the time of constructor execution a SelectedClient is null. I want my dbset to contain LogEntries only by selected client (both of clients and logs tables in db have a client_id field). How can i achieve that? I undestand that my constructor code is completely wrong, but i can't figure out what to do in context of 'pure MVVM' approach. Please help me if you can.

Answer 1

If I get you right this is the way I would do it:

1) Create new instances of the ObserableCollections in the constructor so the binding will be established.

2) move the code to fill the LogEntry List to the Setter of SelectedClient

 public Client SelectedClient
{
    get { return GetValue<Client>(SelectedClientProperty); }
    set { 
             SetValue(SelectedClientProperty, value);
             if(value == null)
             {
                 return;
             }
             var qry = Global.DbContext.Logs.Where(c => c.client_id ==   value.client_id);
            qry.Load();
            LogEntries.Clear();
            foreach(var entry in qry)
            {
                LogEntries.Add(entry);
            }
        }
}