Entity Framework 4.0 TPH : Part 2

Table Inheritance in Entity Framework

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.

1. THP - Table Per Hierarchy
2. TPT - Table Per Type
3. TPC - Table Per Concrete Class

Table Per Hierarchy

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) which are inherited from the base table.

Inheritance using TPH

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. In the conceptual model define only non-inherited properties for derived types. The discriminator column (which acts as 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 value like 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 "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 Entity Model from 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 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 condition.

Step 7 : Make Base Entity abstract.

Step 8 : Now update EDMX manually.

1. Storage Model -In storage Model, Define Entity type, Key and properties which are same as database. There is no definition for derived in storage model.
   
   Coding
   
   <EntityType Name="Departments">
             <Key>
               <PropertyRef Name="DepartmentId" />
             </Key>
             <Property Name="DepartmentId" Type="int" Nullable="false" />
             <Property Name="DepartmentCode" Type="varchar" Nullable="false" MaxLength="25" />
             <Property Name="DepartmentName" Type="varchar" Nullable="false" MaxLength="100" />
             <Property Name="IsActive" Type="bit" Nullable="false" />
             <Property Name="DepartmentType" Type="int" Nullable="false" />
   </EntityType>
    
2. Conceptual Model- In conceptual Model, define base entity without discriminator column and also define derived type.
   
   Coding
   
   <EntityType Name="Department" Abstract="true">
             <Key>
               <PropertyRef Name="DepartmentId" />
             </Key>
             <Property Type="Int32" Name="DepartmentId" Nullable="false" />
             <Property Type="String" Name="DepartmentCode" Nullable="false" MaxLength="25" FixedLength="false" Unicode="false" />
             <Property Type="String" Name="DepartmentName" Nullable="false" MaxLength="100" FixedLength="false" Unicode="false" />
             <Property Type="Boolean" Name="IsActive" Nullable="false" />
             <NavigationProperty Name="Employees" Relationship="AdventureWorksModel.FK_Employees_Department" FromRole="Department" ToRole="Employee" />
   </EntityType>
   <EntityType Name="SalesDepartment" BaseType="AdventureWorksModel.Department">
           </EntityType>
    

3. Mapping Model- the Mapping Model contains the entity type mapping i.e. mapping between conceptual model and storage model.
   
   Example
   
   In this example, mapping between Department and Sales Department are defined in the same EntitySetMapping. The IsTypeOf keyword is used to indicate the type of entity set (i.e. base entity or derived entity). In the Condition tag is a condition in which a sale Department is identified from Department.
   
   <EntitySetMapping Name="Departments" >
               <EntityTypeMapping TypeName="IsTypeOf(AdventureWorksModel.Department)" >
                 <MappingFragment StoreEntitySet="Departments" >
                   <ScalarProperty Name="DepartmentId" ColumnName="DepartmentId" />
                   <ScalarProperty Name="IsActive" ColumnName="IsActive" />
                   <ScalarProperty Name="DepartmentName" ColumnName="DepartmentName" />
                   <ScalarProperty Name="DepartmentCode" ColumnName="DepartmentCode" />
                 </MappingFragment>
               </EntityTypeMapping>
               <EntityTypeMapping TypeName="IsTypeOf(AdventureWorksModel.SalesDepartment)">
                 <MappingFragment StoreEntitySet="Departments">
                   <ScalarProperty Name="DepartmentId" ColumnName="DepartmentId" />
                   <Condition ColumnName="DepartmentType" Value="2" />
                 </MappingFragment>
               </EntityTypeMapping>
               <EntityTypeMapping TypeName="IsTypeOf(AdventureWorksModel.AdminDepartment)">
                 <MappingFragment StoreEntitySet="Departments">
                   <ScalarProperty Name="DepartmentId" ColumnName="DepartmentId" />
                   <Condition ColumnName="DepartmentType" Value="1" />
                 </MappingFragment>
               </EntityTypeMapping>
     </EntitySetMapping>

 In the next article we will learn about TPT (Table per Type) and how to add it in EDMX.