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
- 23rd July, 2010: Initial post