Entity Framework: Insists on adding new entity in many-to-many instead of re-using existing FK

I have got a many to many relationship, briefly
Cases -----< CaseSubjectRelationships >------ CaseSubjects

More fully: Cases(ID, CaseTypeID, .......)
CaseSubjects(ID, DisplayName, CRMSPIN)
CaseSubjectsRelationships(CaseID, SubjectID, PrimarySubject, RelationToCase, ...)

In my many-to-many link table are additional properties relating to the subject's association with the specific case - such as, start date, end date, free-text relationship to case (observer, creator, etc)

An Entity Framework data model has been created - ASP.NET version 4.0

I have a WCF service with a method called CreateNewCase which accepts as its parameter a Case object (an entity created by the Entity Framework) - its job is to save the case into the database.

The WCF service is invoked by a third party tool. Here is the SOAP sent:

<s:Envelope xmlns:s="http://schemas.xmlsoap.org/soap/envelope/">
    <s:Body>
        <CreateNewCase xmlns="http://tempuri.org/">
            <c xmlns:a="http://schemas.datacontract.org/2004/07/CAMSModel">
                <a:CaseSubjectsRelationships>
                    <a:CaseSubjectsRelationship>
                        <a:CaseSubject>
                            <a:CRMSPIN>601</a:CRMSPIN>
                            <a:DisplayName>Fred Flintstone</a:DisplayName>
                        </a:CaseSubject>
                        <a:PrimarySubject>true</a:PrimarySubject>
                        <a:RelationToCase>Interested</a:RelationToCase>
                        <a:StartDate>2011-07-12T00:00:00</a:StartDate>
                    </a:CaseSubjectsRelationship>
                    <a:CaseSubjectsRelationship>
                        <a:CaseSubject>
                            <a:CRMSPIN>602</a:CRMSPIN>
                            <a:DisplayName>Barney Rubble</a:DisplayName>
                        </a:CaseSubject>
                        <a:RelationToCase>Observer</a:RelationToCase>
                        <a:StartDate>2011-07-12T00:00:00</a:StartDate>
                    </a:CaseSubjectsRelationship>
                </a:CaseSubjectsRelationships>
                <a:CaseType>
                    <a:Identifier>Change of Occupier</a:Identifier>
                </a:CaseType>
                <a:Description>Case description</a:Description>
                <a:Priority>5</a:Priority>
                <a:QueueIdentifier>Queue One</a:QueueIdentifier>
                <a:Title>Case title</a:Title>
            </c>
        </CreateNewCase>
    </s:Body>
</s:Envelope>

The WCF engine deserializes this into a Case entity for me correctly and when I look in the debugger everything is set up properly.

What I want to do, is only create a new CaseSubject if there is not already an entry in the database with that CRMSPIN specified (CRMSPIN is a reference number from a central customer database)

So, in the below example, I want to see if I already have an entry in CaseSubjects for somebody with CRMSPIN 601 and if I do, I don't want to create another (duplicate) entry but instead make the new case link to the existing subject (although a new row will need, obviously, need creating in CaseSubjectsRelationships with the specific 'additional' information such as relationship etc)

Here is the .NET code I have tried to do this.

Public Class CamsService
    Implements ICamsService

    Public Function CreateNewCase(c As CAMSModel.Case) As String Implements ICamsService.CreateNewCase

        Using ctx As New CAMSEntities
            ' Find the case type '
            Dim ct = ctx.CaseTypes.SingleOrDefault(Function(x) x.Identifier.ToUpper = c.CaseType.Identifier.ToUpper)

            ' Give an error if no such case type '
            If ct Is Nothing Then
                Throw New CaseTypeInvalidException(String.Format("The case type {0} is not valid.", c.CaseType.Identifier.ToString))
            End If

            ' Set the case type based on that found in database: '
            c.CaseType = ct

            For Each csr In c.CaseSubjectsRelationships
                Dim spin As String = csr.CaseSubject.CRMSPIN
                Dim s As CaseSubject = ctx.CaseSubjects.SingleOrDefault(Function(x) x.CRMSPIN = spin)

                If Not s Is Nothing Then
                    ' The subject has been found based on CRMSPIN so set the subject in the relationship '
                    csr.CaseSubject = s
                End If
            Next

            c.CreationChannel = "Web service"
            c.CreationDate = Now.Date

            ' Save it '
            ctx.AddToCases(c)
            ctx.SaveChanges()
        End Using

        ' Return the case reference '
        Return c.ID.ToString
    End Function
End Class

As you can see, instead the For Each loop, I try to get a subject based on the CRMSPIN and if I get something, then I update the "CaseSubject" entity. (I have also tried csr.SubjectID = s.ID instead of setting the whole entity and also I have tried setting them both!).

However, even when putting a breakpoint on the ctx.SaveChanges() line and looking at how the subjects are set up and seeing in the debugger that it looks fine, it is always creating a new row in the CaseSubjects table.

I can see in principle this should work - you'll see I've done exactly the same thing for Case Type - I have picked the identifier sent in the XML, found the entity with that identifier via the context, then changed the case's .CaseType to the entity I found. When it saves, it works perfectly and as-expected and with no duplicated rows.

I'm just having trouble trying to apply the same theory to one side of a many-to-many relationship.

Here are some (hopefully relevant) extracts from the .edmx

<EntitySet Name="Cases" EntityType="CAMSModel.Store.Cases" store:Type="Tables" Schema="dbo" />
          <EntitySet Name="CaseSubjects" EntityType="CAMSModel.Store.CaseSubjects" store:Type="Tables" Schema="dbo" />
          <EntitySet Name="CaseSubjectsRelationships" EntityType="CAMSModel.Store.CaseSubjectsRelationships" store:Type="Tables" Schema="dbo" />



 <AssociationSet Name="FK_CaseSubjectsRelationships_Cases" Association="CAMSModel.Store.FK_CaseSubjectsRelationships_Cases">
            <End Role="Cases" EntitySet="Cases" />
            <End Role="CaseSubjectsRelationships" EntitySet="CaseSubjectsRelationships" />
          </AssociationSet>
          <AssociationSet Name="FK_CaseSubjectsRelationships_CaseSubjects" Association="CAMSModel.Store.FK_CaseSubjectsRelationships_CaseSubjects">
            <End Role="CaseSubjects" EntitySet="CaseSubjects" />
            <End Role="CaseSubjectsRelationships" EntitySet="CaseSubjectsRelationships" />
          </AssociationSet>

EDIT: The property setters for the CaseSubject property of the CaseSubjectsRelationships object:

/// <summary>
/// No Metadata Documentation available.
/// </summary>
<XmlIgnoreAttribute()>
<SoapIgnoreAttribute()>
<DataMemberAttribute()>
<EdmRelationshipNavigationPropertyAttribute("CAMSModel", "FK_CaseSubjectsRelationships_CaseSubjects", "CaseSubject")>
Public Property CaseSubject() As CaseSubject
    Get
        Return CType(Me, IEntityWithRelationships).RelationshipManager.GetRelatedReference(Of CaseSubject)("CAMSModel.FK_CaseSubjectsRelationships_CaseSubjects", "CaseSubject").Value
    End Get
    Set
        CType(Me, IEntityWithRelationships).RelationshipManager.GetRelatedReference(Of CaseSubject)("CAMSModel.FK_CaseSubjectsRelationships_CaseSubjects", "CaseSubject").Value = value
    End Set
End Property