Entity Data Model: Inheritance

The Entity Data Model (EDM) supports inheritance for entity types. Inheritance in the EDM is similar to inheritance for classes in object-oriented programming languages. Like with classes in object-oriented languages, in a conceptual model you can define an entity type (a derived type) that inherits from another entity type (the base type). However, unlike classes in object-oriented programming, in a conceptual model the derived type always inherits all the properties and navigation properties of the base type. You cannot override inherited properties in a derived type.

In a conceptual model you can build inheritance hierarchies in which a derived type inherits from another derived type. The type at the top of the hierarchy (the one type in the hierarchy that is not a derived type) is called the root type. In an inheritance hierarchy, the entity key must be defined on the root type.

You cannot build inheritance hierarchies in which a derived type inherits from more than one type. For example, in a conceptual model with a Book entity type, you could define derived types FictionBook and NonFictionBook that each inherit from Book. However, you could not then define a type that inherits from both the FictionBook and NonFictionBook types.

Example

The following diagram shows a conceptual model with four entity types: Book, FictionBook, Publisher, and Author. The FictionBook entity type is a derived type, inheriting from the Book entity type. The FictionBook type inherits the ISBN (Key), Title, and Revision properties, and defines an additional property called Genre.

The ADO.NET Entity Framework uses a domain-specific language (DSL) called conceptual schema definition language (CSDL) to define conceptual models. The following CSDL defines an entity type, FictionBook, that inherits from the Book type (as in the diagram above):

<EntityType Name="FictionBook" BaseType="BooksModel.Book" >
<Property Type="String" Name="Genre" Nullable="false" />
</EntityType>