Table Per Hierarchy Inheritance 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:

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:

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.

Make Person entity their base type.

After that, the model should look like:

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.

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:

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:

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.

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

• 23^rd July, 2010: Initial post