Table Per Hierarchy Inheritance in Entity Framework
In the second
tutorials I’m going to
write about the
Table Per Hierarchy
mapping. If you want to read
about the first mapping I showed go to the Table Per Type post from
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.
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.
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.
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:
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:
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.
Test the inheritance mapping. The following code will print the
number of people in the database and also the number of professors
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.