How to: Change Inheritance Mapping

• Aug 24, 2020
• 6 minutes to read

XPO fully supports inheritance and polymorphism of persistent objects, that is you can retrieve a collection of base objects and then deal with the actual instances of concrete classes.

C#

public class Person: XPObject {
    public string PhoneNumber {
        get { return fPhoneNumber; }
        set { SetPropertyValue(nameof(PhoneNumber), ref fPhoneNumber, value); }
    }
    string fPhoneNumber = "";

}

public class Contact: Person {
    public string FirstName {
        get { return fFirstName; }
        set { SetPropertyValue(nameof(FirstName), ref fFirstName, value); }
    }
    string fFirstName = "";

    public string LastName {
        get { return fLastName; }
        set { SetPropertyValue(nameof(LastName), ref fLastName, value); }
    }
    string fLastName = "";

    public string Email {
        get { return fEmail; }
        set { SetPropertyValue(nameof(Email), ref fEmail, value); }
    }
    string fEmail = "";

    public Company Employer {
        get { return fEmployer; }
        set { SetPropertyValue(nameof(Employer), ref fEmployer, value); }
    }
    Company fEmployer;

    // ...
}

public class Company: Person {
    public string Name {
        get { return fName; }
        set { SetPropertyValue(nameof(Name), ref fName, value); }
    }
    string fName = "";

    public string WebSite {
        get { return fWebSite; }
        set { SetPropertyValue(nameof(WebSite), ref fWebSite, value); }
    }
    string fWebSite = "";

    // ...
}

VB.NET

Public Class Person : Inherits XPObject
    Public Property PhoneNumber() As String
        Get
            Return fPhoneNumber
        End Get
        Set(ByVal value as String)
            SetPropertyValue(NameOf(PhoneNumber), fPhoneNumber, value)
        End Set
    End Property
    Private fPhoneNumber As String = ""

End Class

Public Class Contact : Inherits Person
    Public Property FirstName() As String
        Get
            Return fFirstName
        End Get
        Set(ByVal value as String)
            SetPropertyValue(NameOf(FirstName), fFirstName, value)
        End Set
    End Property
    Private fFirstName As String = ""

    Public Property LastName() As String
        Get
            Return fLastName
        End Get
        Set(ByVal value as String)
            SetPropertyValue(NameOf(LastName), fLastName, value)
        End Set
    End Property
    Private fLastName As String = ""

    Public Property Email() As String
        Get
            Return fEmail
        End Get
        Set(ByVal value as String)
            SetPropertyValue(NameOf(Email), fEmail, value)
        End Set
    End Property
    Private fEmail As String = ""

    Public Property Employer() As Company
        Get
            Return fEmployer
        End Get
        Set(ByVal value as Company)
            SetPropertyValue(NameOf(Employer), fEmployer, value)
        End Set
    End Property
    Private fEmployer As Company = Nothing

    ' ...
End Class

Public Class Company : Inherits Person
    Public Property Name() As String
        Get
            Return fName
        End Get
        Set(ByVal value as String)
            SetPropertyValue(NameOf(Name), fName, value)
        End Set
    End Property
    Private fName As String = ""

    Public Property WebSite() As String
        Get
            Return fWebSite
        End Get
        Set(ByVal value as String)
            SetPropertyValue(NameOf(WebSite), fWebSite, value)
        End Set
    End Property
    Private fWebSite As String = ""

    ' ...
End Class

In the code example above, the Contact and Company classes are based on the Person class. By default, XPO will store descendant-specific properties in separate tables and it will automatically build table joins to retrieve object data:

If you want to store the properties of a descendant class in a base class table, you can change the inheritance mapping type to ParentTable. In this instance, XPO will store all properties from all the descendants in the same table.

C#

public class Person: XPObject {
    public string PhoneNumber {
        get { return fPhoneNumber; }
        set { SetPropertyValue(nameof(PhoneNumber), ref fPhoneNumber, value); }
    }
    string fPhoneNumber = "";

}

[MapInheritance(MapInheritanceType.ParentTable)]
public class Contact: Person {
    public string FirstName {
        get { return fFirstName; }
        set { SetPropertyValue(nameof(FirstName), ref fFirstName, value); }
    }
    string fFirstName = "";

    public string LastName {
        get { return fLastName; }
        set { SetPropertyValue(nameof(LastName), ref fLastName, value); }
    }
    string fLastName = "";

    public string Email {
        get { return fEmail; }
        set { SetPropertyValue(nameof(Email), ref fEmail, value); }
    }
    string fEmail = "";

    public Company Employer {
        get { return fEmployer; }
        set { SetPropertyValue(nameof(Employer), ref fEmployer, value); }
    }
    Company fEmployer;

    // ...
}

[MapInheritance(MapInheritanceType.ParentTable)]
public class Company : Person {
    public string Name {
        get { return fName; }
        set { SetPropertyValue(nameof(Name), ref fName, value); }
    }
    string fName = "";

    public string WebSite {
        get { return fWebSite; }
        set { SetPropertyValue(nameof(WebSite), ref fWebSite, value); }
    }
    string fWebSite = "";

    // ...
}

VB.NET

Public Class Person : Inherits XPObject
    Public Property PhoneNumber() As String
        Get
            Return fPhoneNumber
        End Get
        Set(ByVal value as String)
            SetPropertyValue(NameOf(PhoneNumber), fPhoneNumber, value)
        End Set
    End Property
    Private fPhoneNumber As String = ""

End Class

<MapInheritance(MapInheritanceType.ParentTable)> _
Public Class Contact : Inherits Person
    Public Property FirstName() As String
        Get
            Return fFirstName
        End Get
        Set(ByVal value as String)
            SetPropertyValue(NameOf(FirstName), fFirstName, value)
        End Set
    End Property
    Private fFirstName As String = ""

    Public Property LastName() As String
        Get
            Return fLastName
        End Get
        Set(ByVal value as String)
            SetPropertyValue(NameOf(LastName), fLastName, value)
        End Set
    End Property
    Private fLastName As String = ""

    Public Property Email() As String
        Get
            Return fEmail
        End Get
        Set(ByVal value as String)
            SetPropertyValue(NameOf(Email), fEmail, value)
        End Set
    End Property
    Private fEmail As String = ""

    Public Property Employer() As Company
        Get
            Return fEmployer
        End Get
        Set(ByVal value as Company)
            SetPropertyValue(NameOf(Employer), fEmployer, value)
        End Set
    End Property
    Private fEmployer As Company = Nothing

    ' ...
End Class

<MapInheritance(MapInheritanceType.ParentTable)> _
Public Class Company : Inherits Person
    Public Property Name() As String
        Get
            Return fName
        End Get
        Set(ByVal value as String)
            SetPropertyValue(NameOf(Name), fName, value)
        End Set
    End Property
    Private fName As String = ""

    Public Property WebSite() As String
        Get
            Return fWebSite
        End Get
        Set(ByVal value as String)
            SetPropertyValue(NameOf(WebSite), fWebSite, value)
        End Set
    End Property
    Private fWebSite As String = ""

    ' ...
End Class

In the code example above, XPO will use a single table to store the inheritance hierarchy:

You can specify different inheritance mapping types for members of the same hierarchy. This technique gives you a great degree of control over data store layout in your database.

For developers working with persistent objects, both types of inheritance mapping look similar - it does not affect object behavior in any way. The only reason you will want to deal with the inheritance mapping type attribute is to fine-tune the database object store, segmentation and performance.

Note

By making the base class non-persistent you can implement inheritance mapping with which all the attributes, including inherited, are stored in the table which corresponds to a descendant class. The only exception of this implementation is that you cannot reference the base class in other persistent objects.