How to: Change Inheritance Mapping

• Nov 21, 2018
• 3 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 = "";
}

public class Contact: Person {
    public string FirstName = "";
    public string LastName = "";
    public string Email = "";
    public Company Employer;
    // ...
}

public class Company: Person {
    public string Name = "";
    public string WebSite = "";
    // ...
}

VB.NET

Public Class Person : Inherits XPObject
    Public PhoneNumber As String = ""
End Class

Public Class Contact : Inherits Person
    Public FirstName As String = ""
    Public LastName As String = ""
    Public Email As String = ""
    Public Employer As Company = Nothing
    ' ...
End Class

Public Class Company : Inherits Person
    Public Name As String = ""
    Public WebSite 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 = "";
}

[MapInheritance(MapInheritanceType.ParentTable)]
public class Contact: Person {
    public string FirstName = "";
    public string LastName = "";
    public string Email = "";
    public Company Employer;
    // ...
}

[MapInheritance(MapInheritanceType.ParentTable)]
public class Company : Person {
    public string Name = "";
    public string WebSite = "";
    // ...
}

VB.NET

Public Class Person : Inherits XPObject
    Public PhoneNumber As String = ""
End Class

<MapInheritance(MapInheritanceType.ParentTable)> _
Public Class Contact : Inherits Person
    Public FirstName As String = ""
    Public LastName As String = ""
    Public Email As String = ""
    Public Employer As Company = Nothing
    ' ...
End Class

<MapInheritance(MapInheritanceType.ParentTable)> _
Public Class Company : Inherits Person
    Public Name As String = ""
    Public WebSite 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.