Table Per Hierarchy Inheritance in Entity Framework

Table Per Hierarchy Inheritance in Entity Framework

In the second
inheritance mappings
tutorials 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 create 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 belong 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 exactly 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 it’s 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());

}

Console.ReadLine();

Summary

Lets sum up, in the post 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.
In the last post in this series I’m going to explain what is
Table Per Concrete Type inheritance and how to use it in EF.