How to: Implement One-to-One Relationships

• Oct 29, 2020
• 3 minutes to read

There are three types of relationships between objects. The type of a relationship that is created depends upon how related objects are defined. To learn about a particular relationship type, click a corresponding link below.

• One-to-Many Relationships
• One-to-One Relationships
• Many-to-Many 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.

C#

// Represents the Building class which refers to the building's owner.
public class Building : XPObject {
    Person owner = null;
    public Person Owner {
        get { return owner; }
        set {
            if(owner == value)
                return;

            // Store a reference to the former owner.
            Person prevOwner = owner;
            owner = value;

            if(IsLoading) return;

            // Remove an owner's reference to this building, if exists.
            if(prevOwner != null && prevOwner.House == this)
                prevOwner.House = null;

            // Specify that the building is a new owner's house.
            if(owner != null)
                owner.House = this;
            OnChanged(nameof(Owner));
        }
    }
}

// Represents the Person class which refers to the person's house.
public class Person : XPObject {
    Building house = null;
    public Building House {
        get { return house; }
        set {
            if(house == value)
                return;

            // Store a reference to the person's former house.
            Building prevHouse = house;
            house = value;

            if(IsLoading) return;

            // Remove a reference to the house's owner, if the person is its owner.
            if(prevHouse != null && prevHouse.Owner == this)
                prevHouse.Owner = null;

            // Specify the person as a new owner of the house.
            if(house != null)
                house.Owner = this;

            OnChanged(nameof(House));
        }
    }
}

VB.NET

' Represents the Building class which refers to the building's owner.
Public Class Building : Inherits XPObject
    Dim _owner As Person = Nothing
    Public Property Owner() As Person
        Get
            Return _owner
        End Get
        Set(ByVal Value As Person)
            If _owner Is Value Then
                Return
            End If

            ' Store a reference to the former owner.
            Dim prevOwner As Person = _owner
            _owner = Value

            If IsLoading Then Return

            ' Remove an owner's reference to this building, if exists.
            If Not (prevOwner Is Nothing) AndAlso prevOwner.House Is Me Then
                prevOwner.House = Nothing
            End If

            ' Specify that the building is a new owner's house.
            If Not (_owner Is Nothing) Then
                _owner.House = Me
            End If

            OnChanged(NameOf(Owner))

        End Set
    End Property
End Class

' Represents the Person class which refers to the person's house.
Public Class Person : Inherits XPObject
    Dim _house As Building = Nothing
    Public Property House() As Building
        Get
            Return _house
        End Get
        Set(ByVal Value As Building)
            If _house Is Value Then
                Return
            End If

           ' Store a reference to the person's former house.
            Dim prevHouse As Building = _house
            _house = Value

            If IsLoading Then Return

            ' Remove a reference to the house's owner, if the person is its owner.
            If Not (prevHouse Is Nothing) AndAlso prevHouse.Owner Is Me Then
                prevHouse.Owner = Nothing
            End If

            ' Specify the person as a new owner of the house.
            If Not (_house Is Nothing) Then
                _house.Owner = Me
            End If

            OnChanged(NameOf(House))
        End Set
    End Property
End Class