How to Bind a Combo Box so that it retrieves and display content corresponding to the Id in a link table and populates itself with the data in the main table?

Question

I am developing a desktop application in Wpf using MVVM and Entity Frameworks. I have the following tables:
1. Party (PartyId, Name)
2. Case (CaseId, CaseNo)
3. Petitioner (CaseId, PartyId) ............. Link Table
I am completely new to .Net and to begin with I download Microsoft's sample application and
following the pattern I have been successful in creating several tabs. The problem started only when I wanted to implement many-to-many relationship. The sample application has not covered the scenario where there can be a any-to-many relationship. However
with the help of MSDN forum I came to know about a link table and managed to solve entity framework issues pertaining to many-to-many relationship. Here is the screenshot of my application to show you what I have achieved so far.
And now the problem I want the forum to address is how to bind a combo box so that it retrieves Party.Name for the corresponding PartyId in the Link Table and also I want to populate it with Party.Name so that
users can choose one from the dropdown list to add or edit the petitioner.

Answer 1

Hello Barry,
Thanks a lot for responding to my query. As I am completely new to .Net and following the pattern of Microsoft's Employee Tracker sample it seems difficult to clearly understand the concept and implement it in a scenario which is different than what is in
the sample available at the link you supplied.
To get the idea of the thing here is my code behind of a view vBoxPetitioner:
<UserControl x:Class="CCIS.View.Case.vBoxPetitioner"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:v="clr-namespace:CCIS.View.Case"
xmlns:vm="clr-namespace:CCIS.ViewModel.Case"
mc:Ignorable="d"
d:DesignWidth="300"
d:DesignHeight="200">
<UserControl.Resources>
<DataTemplate DataType="{x:Type vm:vmPetitioner}">
<v:vPetitioner Margin="0,2,0,0" />
</DataTemplate>
</UserControl.Resources>
<Grid>
<HeaderedContentControl>
<HeaderedContentControl.Header>
<StackPanel Orientation="Horizontal" HorizontalAlignment="Right">
<TextBlock Margin="2">
<Hyperlink Command="{Binding Path=AddPetitionerCommand}">Add Petitioner</Hyperlink>
| <Hyperlink Command="{Binding Path=DeletePetitionerCommand}">Delete</Hyperlink>
</TextBlock>
</StackPanel>
</HeaderedContentControl.Header>
<ListBox BorderThickness="0" SelectedItem="{Binding Path=CurrentPetitioner, Mode=TwoWay}" ItemsSource="{Binding Path=tblParties}" />
</HeaderedContentControl>
</Grid>
</UserControl>
This part is working fine as it loads another view that is vPetioner perfectly in the manner I want it to be.
Here is the code of vmPetitioner, a ViewModel:
Imports Microsoft.VisualBasic
Imports System.Collections.ObjectModel
Imports System
Imports CCIS.Model.Party
Namespace CCIS.ViewModel.Case
''' <summary>
''' ViewModel of an individual Email
''' </summary>
Public Class vmPetitioner
Inherits vmParty
''' <summary>
''' The Email object backing this ViewModel
''' </summary>
Private petitioner As tblParty
''' <summary>
''' Initializes a new instance of the EmailViewModel class.
''' </summary>
''' <param name="detail">The underlying Email this ViewModel is to be based on</param>
Public Sub New(ByVal detail As tblParty)
If detail Is Nothing Then
Throw New ArgumentNullException("detail")
End If
Me.petitioner = detail
End Sub
''' <summary>
''' Gets the underlying Email this ViewModel is based on
''' </summary>
Public Overrides ReadOnly Property Model() As tblParty
Get
Return Me.petitioner
End Get
End Property
''' <summary>
''' Gets or sets the actual email address
''' </summary>
Public Property fldPartyId() As String
Get
Return Me.petitioner.fldPartyId
End Get
Set(ByVal value As String)
Me.petitioner.fldPartyId = value
Me.OnPropertyChanged("fldPartyId")
End Set
End Property
End Class
End Namespace
And below is the ViewMode vmParty which vmPetitioner Inherits:
Imports Microsoft.VisualBasic
Imports System
Imports System.Collections.Generic
Imports CCIS.Model.Case
Imports CCIS.Model.Party
Imports CCIS.ViewModel.Helpers
Namespace CCIS.ViewModel.Case
''' <summary>
''' Common functionality for ViewModels of an individual ContactDetail
''' </summary>
Public MustInherit Class vmParty
Inherits ViewModelBase
''' <summary>
''' Gets the underlying ContactDetail this ViewModel is based on
''' </summary>
Public MustOverride ReadOnly Property Model() As tblParty
'''' <summary>
'''' Gets the underlying ContactDetail this ViewModel is based on
'''' </summary>
'Public MustOverride ReadOnly Property Model() As tblAdvocate
''' <summary>
''' Gets or sets the name of this department
''' </summary>
Public Property fldName() As String
Get
Return Me.Model.fldName
End Get
Set(ByVal value As String)
Me.Model.fldName = value
Me.OnPropertyChanged("fldName")
End Set
End Property
''' <summary>
''' Constructs a view model to represent the supplied ContactDetail
''' </summary>
''' <param name="detail">The detail to build a ViewModel for</param>
''' <returns>The constructed ViewModel, null if one can't be built</returns>
Public Shared Function BuildViewModel(ByVal detail As tblParty) As vmParty
If detail Is Nothing Then
Throw New ArgumentNullException("detail")
End If
Dim e As tblParty = TryCast(detail, tblParty)
If e IsNot Nothing Then
Return New vmPetitioner(e)
End If
Return Nothing
End Function
End Class
End Namespace
And final the code behind of the view vPetitioner:
<UserControl x:Class="CCIS.View.Case.vPetitioner"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:vm="clr-namespace:CCIS.ViewModel.Case"
mc:Ignorable="d"
Width="300">
<UserControl.Resources>
<ResourceDictionary Source=".\CompactFormStyles.xaml" />
</UserControl.Resources>
<Grid>
<Border Style="{StaticResource DetailBorder}">
<Grid>
<Grid.ColumnDefinitions>
<ColumnDefinition Width="Auto" />
<ColumnDefinition Width="*" />
</Grid.ColumnDefinitions>
<TextBlock Grid.Column="0" Text="Petitioner:" />
<ComboBox Grid.Column="1" Width="240" SelectedValuePath="." SelectedItem="{Binding Path=tblParty}" ItemsSource="{Binding Path=PetitionerLookup}" DisplayMemberPath="fldName" />
</Grid>
</Border>
</Grid>
</UserControl>
The problem, presumably, seems to be is that the binding path "PetitionerLookup" of the ItemSource of the Combo box in the view vPetitioner exists in a different ViewModel vmCase which serves as an ObservableCollection for MainViewModel. Therefore,
what I need to Know is how to route the binding path if it exists in a different ViewModel?
Sir, I look forward to your early reply bringing a workable solution to the problem I face. 
Warm Regards,
Arun