Adding Abstract Entity in Entity Framework

This post will show how to create an Abstract entity and its derived entities in Entity Framework 4.1.

Suppose we have an 'Employee' table as shown below that is set as a table to store two different types of employees, namely staffs and managers. For simplicity; staffs are under managers, both staffs and managers have 'FirstName' and 'LastName', only staffs have 'DeskNumbers' while managers have 'OfficeRoomNumbers'. They are differentiate by 'Type' flag. We could see that 'Table per Hierarchy' (TPH) style is used here.

We would like to create these mappings of staffs and managers to the 'Employee' table in Entity Framework designer. To implement this 'Table per Hierarchy' style, we will create an abstract Employee entity and have Staff and Manager entities as the derived/child entities from Employee entity.

Assume we create Employee entity from scratch:
1. Right click an empty space on designer
2. Select Add > Entity
3. Type 'Employee' as Entity name
4. Leave Base type as '(None)'
5. On the 'Key Property' section, type 'EmployeeID' as the Property name. This is the same name as the primary key column name in the database table
6. Click 'OK' then a new entity is added

7. A new entity called 'Employee' is added
8. Right click the entity then select Properties
9. Change 'Abstract' value to 'True.

Then we create the child entities:
1. Right click an empty space on designer
2. Select Add > Entity
3. Type 'Staff' as Entity name
4. On Base type dropdown select' Employee'. This will make 'Key Property' section disabled.

5. Click 'OK' then the new entity is added
6. Repeat the same process to add 'Manager' entity

Next we need to add properties that are common to both derived entities on the abstract entity. In this case we need to add 'FirstName' and 'LastName' as Scalar Properties on Employee entity. Make sure to modify the 'Type' and 'MaxLength' values of the properties according to their data types in database.

After we add the common properties, we need to add properties that are specific to the derived entities. Add 'DeskNumber' Scalar Property to Staff entity and 'OfficeRoomNumber' Scalar Property to Manager entity. Right click each of the newly added Scalar Properties then select Properties. Modify the 'Type' of the properties and make sure the 'Nullable' value is set to 'True'.

After doing all of those, we will have this:

Finally we need to map the entities to the table in database. First we map the abstract entity.
1. Right click 'Employee' entity then select Table Mapping
2. Click '<Add a Table or View>' then select 'Employee' from dropdown
3. Then you will see under 'Column Mappings' all the columns from 'Employee' table in the database are displayed on the left side, while the entity properties are displayed on the right side. All matched properties are automatically mapped.

The derived entities need to be mapped as well. They will be mapped to the same table as the abstract entity.
1. Right click 'Staff' entity then select Table Mapping
2. Click '<Add a Table or View>' then select 'Employee' from dropdown
3. Click '<Add a Condition>' then select 'Type' from dropdown
4. Type 'S' as the ' When Type' value

5. Do the same process with 'Manager' entity, however the 'When Type' value will be 'M'