Entity Framework 4.0 TPH : Part 2

In OOP, we use inheritance to reduce unnecessary code. We can also archive inheritance in Entity Framework. Entity Framework supports three different types of inheritance.

Table-per-hierarchy inheritance uses one database table to maintain data for more than one entity types in an inheritance hierarchy in Entity Framework. In other words, there are many entity sets in EF for a single database table. In TPH, the base table or entity could be created and it has a number of derived entities (must be more than one) that are inherited from the base table.

Define entity set in Conceptual Model that will act as a base entity type and others act as derived types. A derived entity type must inherit from a base type and in the conceptual model set Base Type Attribute. The conceptual model defines only non-inherited properties for derived types. The discriminator column (which acts as a separator for base type) is mapped as part of a condition in the Mapping Model, so there can be no corresponding property on any entity types in the hierarchy. The exception to this rule is when the condition uses an Is Null or Is Not Null comparison. In this case, the discriminator column can have a corresponding property on an entity type if the discriminator column contains more than a value like an integer then the column must be nullable or have a default value to ensure that when the new type is created and saved to the database, the column has some values. Conditions must be used to map each derived type and base type in the hierarchy. There is no mapping or conditions allowed if the base type is abstract. The Base Entity type and derived entity are mapped in the "EntitySetMapping" element in Mapping Model. IsTypeOf keyword is used to set the value of the Type Name attribute on derived types. Use a mapping condition to discriminate between types in the hierarchy. Here are the steps to follow.

Step 1: Create an Entity Model from the database. Step 2: Add new Entity (right-click on Entity designer Add -go to Entity). Step 3: Make derived Entity Like "AdminDepartment" and "Sales Department".

Step 4: Remove discriminator columns from the base type Table "Department". Step 5: Add Table mapping - right-click on Derived type entity and select "Table Mapping". Step 6: Map Table and add a condition.

Step 7: Make a Base Entity abstract.

Step 8: Now update EDMX manually.