XAF : Implement One-to-One Relationships

Let's consider a simple example of a One-to-One relationship, when both classes that participate in the relationship have properties with a reference to the instance of their opposite class. It's necessary to write extra code within the property's setter method for each class that participates in the relationship to ensure the relationship's integrity, i.e. when a new object is assigned to a reference property, the reference to the previous object instance should be cleared and the assigned object should reference the current one.

This technique can be implemented as shown in the following code example.

   1:  ' Represents the Building class which refers to the building's owner. 

   2:  Public Class Building : Inherits XPObject

   3:      Dim _owner As Person = Nothing 

   4:      Public Property Owner() As Person

   5:          Get 

   6:              Return _owner

   7:          End Get 

   8:          Set(ByVal Value As Person)

   9:              If _owner Is Value Then 

  10:                  Return 

  11:              End If 

  12:   

  13:              ' Store a reference to the former owner. 

  14:              Dim prevOwner As Person = _owner

  15:              _owner = Value

  16:   

  17:              If IsLoading Then Return 

  18:   

  19:              ' Remove an owner's reference to this building, if exists. 

  20:              If Not (prevOwner Is Nothing) AndAlso prevOwner.House Is Me Then 

  21:                  prevOwner.House = Nothing 

  22:              End If 

  23:   

  24:              ' Specify that the building is a new owner's house. 

  25:              If Not (_owner Is Nothing) Then 

  26:                  _owner.House = Me 

  27:              End If 

  28:   

  29:              OnChanged("Owner")

  30:   

  31:          End Set 

  32:      End Property 

  33:  End Class 

  34:   

  35:  ' Represents the Person class which refers to the person's house. 

  36:  Public Class Person : Inherits XPObject

  37:      Dim _house As Building = Nothing 

  38:      Public Property House() As Building

  39:          Get 

  40:              Return _house

  41:          End Get 

  42:          Set(ByVal Value As Building)

  43:              If _house Is Value Then 

  44:                  Return 

  45:              End If 

  46:   

  47:             ' Store a reference to the person's former house. 

  48:              Dim prevHouse As Building = _house

  49:              _house = Value

  50:   

  51:              If IsLoading Then Return 

  52:   

  53:              ' Remove a reference to the house's owner, if the person is its owner. 

  54:              If Not (prevHouse Is Nothing) AndAlso prevHouse.Owner Is Me Then 

  55:                  prevHouse.Owner = Nothing 

  56:              End If 

  57:   

  58:              ' Specify the person as a new owner of the house. 

  59:              If Not (_house Is Nothing) Then 

  60:                  _house.Owner = Me 

  61:              End If 

  62:   

  63:              OnChanged("House")

  64:          End Set 

  65:      End Property 

  66:  End Class 

[eXpressApp Framework]