save in different tables in the database based on the selection of a combobox

I have to create a generic viewmodel passing an entity with a one to many relationship. I'll explain: My WIndows:

<Window x:Class="Invoice_Example_Brux.MainWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:invoiceExampleBrux="clr-namespace:Invoice_Example_Brux"
    Title="MainWindow" Height="350" Width="525"><Window.DataContext><invoiceExampleBrux:MainWindowViewModel/></Window.DataContext><Grid><TextBox HorizontalAlignment="Left" Height="23" Margin="174,78,0,0" TextWrapping="Wrap" Text="{Binding MyModel.Name}" VerticalAlignment="Top" Width="120"/><Label Content="Id" HorizontalAlignment="Left" Margin="10,53,0,0" VerticalAlignment="Top"/><TextBox HorizontalAlignment="Left" Height="23" Margin="10,78,0,0" TextWrapping="Wrap" Text="{Binding MyModel.Id}" VerticalAlignment="Top" Width="120"  IsReadOnly="True"/><Label Content="Number" HorizontalAlignment="Left" Margin="322,52,0,0" VerticalAlignment="Top"/><TextBox HorizontalAlignment="Left" Height="23" Margin="322,78,0,0" TextWrapping="Wrap" Text="{Binding MyModel.Number}" VerticalAlignment="Top" Width="120"/><Label Content="Name" HorizontalAlignment="Left" Margin="174,53,0,0" VerticalAlignment="Top"/><Button Content="Save" HorizontalAlignment="Left" Margin="211,288,0,0" VerticalAlignment="Top" Width="75" Command="{Binding SaveCommand}"/><ComboBox
        SelectionChanged="Selector_OnSelectionChanged"
              HorizontalAlignment="Left" Margin="180,38,0,0" VerticalAlignment="Top" Width="120"                  
              DisplayMemberPath="Name"
              ItemsSource="{Binding DocumentType,UpdateSourceTrigger=PropertyChanged}"/><Label Content="Type Document" HorizontalAlignment="Left" Margin="192,12,0,0" VerticalAlignment="Top"/></Grid>

myWindows codebheind:

  namespace Invoice_Example_Brux
{

    public partial class MainWindow
    {
        public MainWindow()
        {
            InitializeComponent();
        }

        private void Selector_OnSelectionChanged(object sender, SelectionChangedEventArgs e)
        {
            var cmb  = sender as ComboBox;
            var selectedItem = cmb.SelectedValue as DocumentType;
            if (selectedItem == null) return;
            var code = selectedItem.Code;
            switch (code)
            {
                case "A":
                    DataContext = new ViewModelGeneric<DocumentA>();
                    break;
                case "B":
                    DataContext = new ViewModelGeneric<DocumentB>();
           break;
                case "C":
                    break;
            }
        }
    }
}

My Entity DocumentA and DocumentB:

public class DocumentA : DocumentGeneral
    {
        public ObservableCollection<DetailDocumentA> DetailDocumentA { get; set; }

    }
   public class DetailDocumentA : DetailDocumentGeneral
    {
    }

  public class DocumentB : DocumentGeneral
    {
        public ObservableCollection<DetailDocumentB> DetailDocumentB { get; set; }

    }
   public class DetailDocumentB : DetailDocumentGeneral
    {
    }

    public class DocumentGeneral 
    {
        public Guid Id { get; set; }
        public string Name { get; set; }
        public string Number { get; set; }
        public string TypeDocument { get; set; }

    }
   public class DetailDocumentGeneral
    {
        public Guid Id { get; set; }
        public string Quantity { get; set; }
        public string Price { get; set; }
        public string Total { get; set; }
    }

My ViewModelGeneric:

public class ViewModelGeneric<T> : ViewModelBase
        where T : DocumentGeneral, new()
    {
        public T MyModel { get; set; }
        public RelayCommand SaveCommand { get; set; }
        public ViewModelGeneric()
        {
            MyModel = new T();
            SaveCommand = new RelayCommand(Save);
        }

        private void Save(object obj)
        {
            if (MyModel.Id == Guid.Empty)
            {
                MyModel.Id = Guid.NewGuid();
            }
//the problme is here. how can I do to fill in the detail of my entity
          /*  MyModel.Detail.Add(new DetailDocumentGeneral
           {
               Price = "50",Quantity = "100",Total = "5000"
           });*/

            using (var ctx = new DocumentContext())
            {
                var document = ctx.Set<T>();
                document.Add(MyModel);
                ctx.SaveChanges();
            }
        }
    }

the problem is that depending on the choice of my combobox I have to go a different entity. But I can not access the respective detailDocument = (

the different documents have the same properties.

 I depending on the choice of the combobox I can save different tables in my db. 

I had solved using automapper. 
I did the mapping table (with relevant details table) attached to the window with the one that I chose from combobx. But if I have 10 documents means doing the mapping of all ten (performance decreases). 

I do not know if this is the right way. and for months I'm looking for a more suitable solution but still nothing to do.