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context.SaveChanges(); } using (var context = new EFRecipesEntities()) { context.ContextOptions.LazyLoadingEnabled = true; foreach (var c in context.Customers) { Console.WriteLine("{0} has {1} order(s)", c.FullName, c.TotalOrders.ToString()); foreach (var o in c.Orders) { Console.WriteLine("\tOrdered on: {0}", o.OrderDate.ToShortDateString()); Console.WriteLine("\tStatus: {0}", o.OrderStatus); Console.WriteLine("\tShip via: {0}\n", o.ShippingType); } } } The code in Listing 15-25 produces the following output: Robert Jones has 2 order(s) Ordered on: 11/19/2009 Status: Shipped Ship via: FedEx
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Ordered on: 12/13/2009 Status: Processing Ship via: UPS
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Julia Stevens has 1 order(s) Ordered on: 10/19/2009 Status: Shipped Ship via: FedEx
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15-10. Mapping an Entity to Multiple Tables
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Problem
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You want to insert an entity into different tables based on a property value.
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Let s say you have two tables: WorkOrder and PriorityWorkOrder. The WorkOrder table contains information about common, standard priority work orders. The PriorityWorkOrder table contains the same work order information, but represents work orders that need immediate attention. The tables might look like those in the database diagram in Figure 15-26.
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Figure 15-26. WorkOrder and PriorityWorkOrder tables To create the model that maps a WorkOrder entity to these two tables, do the following: 1. Add a new ADO.NET Entity Data Model to your project and import the WorkOrder and PriorityWorkOrder tables. Or update an existing model with these tables. Right-click the PriorityWorkOrder entity and select Delete. Select No in the dialog box. This will delete the entity without deleting the underlying store layer representation. Right-click the WorkOrder entity and select Add Scalar Property. Name the new property IsPriority. Select the IsPriority property and change its type to Boolean. Right-click the .edmx file in the Solution Explorer and select Open With Editor. XML
4. 5.
Change the EntitySetMapping in the mapping layer to conditionally map the WorkOrder entity to the WorkOrder or PriorityWorkOrder table, as shown in Listing 15-26.
Listing 15-26. Changes for the .edmx file to conditionally map the WorkOrder entity to either the WorkOrder table or the PriorityWorkOrder table <EntitySetMapping Name="WorkOrders"> <EntityTypeMapping TypeName="EFRecipesModel.WorkOrder"> <MappingFragment StoreEntitySet="WorkOrder">
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<ScalarProperty Name="WorkOrderId" ColumnName="WorkOrderId" <ScalarProperty Name="RequestDate" ColumnName="RequestDate" <ScalarProperty Name="Problem" ColumnName="Problem" /> <Condition Name="IsPriority" Value="false" /> </MappingFragment> <MappingFragment StoreEntitySet="PriorityWorkOrder"> <ScalarProperty Name="WorkOrderId" ColumnName="WorkOrderId" <ScalarProperty Name="RequestDate" ColumnName="RequestDate" <ScalarProperty Name="Problem" ColumnName="Problem" /> <Condition Name="IsPriority" Value="true" /> </MappingFragment> </EntityTypeMapping> </EntitySetMapping>
/> />
/> />
How It Works
To store the priority work orders in the PriorityWorkOrder table and the normal work orders in the WorkOrder table, we added the IsPriority property to the entity and used it to conditionally map the entity to the appropriate table. Normally, each property must be mapped to a column in a table. The exception to this rule is when the property participates in a condition, as IsPriority does, and the property cannot be null, which is also true for the IsPriority property. There is one important yet subtle problem. We have two tables that map to the same entity. This means that we have two tables mapping into a single entity set. We need to guarantee that the WorkOrderId values from the two tables never collide. These values must be unique in the entity set, which means they must be unique across both tables. There are several strategies for managing this. One approach is to assign GUIDs to the WorkOrderId. The approach we take here is to set the WorkOrderId as an integer identity column with the WorkOrder table enumerating odd integers and the PriorityWorkOrder table enumerating even integers. This effectively guarantees that the WorkOrderId will be unique across the tables. The code in Listing 15-27 demonstrates inserting into and retrieving from our model. After running this code, check the content of the WorkOrder and PriorityWorkOrder tables. The WorkOrder table should contain the normal work orders while the PriorityWorkOrder table contains the high priority work orders. Listing 15-27. Inserting into and retrieving from our model using (var context = new EFRecipesEntities()) { var wo1 = new WorkOrder { RequestDate = DateTime.Parse("11/04/09"), Problem = "Printer needs paper in shipping.", IsPriority = false }; var wo2 = new WorkOrder { RequestDate = DateTime.Parse("11/04/09"), Problem = "Main site database server is down!", IsPriority = true }; var wo3 = new WorkOrder { RequestDate = DateTime.Parse("11/04/09"), Problem = "Backup job complete, remove tape.", IsPriority = false }; context.WorkOrders.AddObject(wo1); context.WorkOrders.AddObject(wo2);
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