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Table Per Hierarchy Inheritance in Entity Framework

, 23 Jul 2010
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The article explains what Table Per Hierarchy Inheritance is and how to apply it in Entity Framework.

Introduction

In the second inheritance mappings tutorial, I’m going to write about the Table Per Hierarchy (TPH) inheritance mapping. If you want to read about the first mapping I showed, go to the Table Per Type post from here.

Table Per Hierarchy Definition

In TPH, the inheritance tree is created through one table only.
The TPH inheritance depends on a conditional mapping which is defined by a condition such as a discriminator database field. The condition is used to define records as different types. For example, in the following database schema the Person table includes a TPH inheritance:

Database Schema

As you can see, the table holds two different fields from two different types: HireDate (which belong to a professor type) and EnrollmentDate (which belongs to student type). The table also includes an integer discriminator field which is called PersonType. When PersonType equals 1, the person type is a professor and when it is 2, the type is student.

TPH Example

The following steps will help you to understand how you can create a TPH inheritance mapping. I’m going to use the exact database from the first figure I showed.

Step 1

The first step is to create the Entity Data Model from the database. Here is the EDM I’m going to use in the example:

Entity Designer Diagram

We are going to use only the Person entity for the TPH demonstration.

Step 2

From the designer surface, use the Add –> Entity and add two entities: Professor and Student.

Add New Entities

Make Person entity their base type.

Add Professor

After that, the model should look like:

Entity Designer Diagram 1

Step 3

Move the HireDate property to the Professor entity and the EnrollmentDate property to the Student entity. Also remove the PersonType property from Person because it is going to be our discriminator field.

Entity Designer Diagram 2

Step 4

In the Mapping Details View map the Student entity and the Professor entity to the Person table. Also map the relevant properties to the fields in the database. The following figure shows how to map the student entity:

Map The Student Entity

Step 5

Add a condition to the entities using the Mapping Details View. The condition should be on the PersonType field and should indicate that if PersonType equals 1, the person is a Professor type and if the PersonType equals 2, the person type is a Student. The following figure shows how to do it in the Student type:
Add Condition

Step 6

Since Person is a base type which is an abstract type, we need to indicate that it is abstract. Point on the Person entity and press F4 to open its properties dialog. In the dialog, turn the abstract flag to True.

Person Properties

Step 7

Test the inheritance mapping. The following code will print the number of people in the database and also the number of professors and students:

using (var context = new SchoolEntities())
{
    var query = from person in context.People
                select person;
    Console.WriteLine("All People: " + query.Count().ToString());

    var query1 = from student in context.People.OfType<Student>()
                 select student;
    Console.WriteLine("Students: " + query1.Count().ToString());

    var query2 = from proffesor in context.People.OfType<Professor>()
                 select proffesor;
    Console.WriteLine("Professors: " + query2.Count().ToString());

}

Summary

Let's sum up. In the article, I explained what is Table Per Hierarchy inheritance and showed how we can create that inheritance in a specific model. The use of TPH is common and sometimes can lead to very big tables which include a lot of fields. Since it is a bad habit to create very big tables, I suggest to use TPH with TPT.

History

  • 23rd July, 2010: Initial post

License

This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

About the Author

Gil Fink
Architect Sela Group
Israel Israel
Gil Fink is an expert in web development and Microsoft web and data platforms. He works as a senior consultant and architect at Sela Group. He is currently consulting for various enterprises and companies, where he helps to develop Web and RIA-based solutions. He conducts lectures and workshops for individuals and enterprises who want to specialize in infrastructure and web development. He is also a co-author of several Microsoft Official Courses and training kits and the founder of Front-End.IL meetup. You can read his publications at his website: http://www.gilfink.net
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GeneralMy vote of 5 Pinmemberzaman_cisco23-Oct-12 1:48 

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