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Roland Jones, Hire date: 8/14/2005, Salary: $61,000.00 Instructor) Steven Curtis, Hire date: 8/23/1992, Salary: $68,200.00 Instructor)
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Students ========== Karen Roberts Bobby McGivens Janis Hettler
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15-5. Sharing Audit Fields Across Multiple Entities
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You have several tables with common audit columns and you want to create a model that simplifies updating these audit columns
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Let s say you have the two tables shown in Figure 15-9. These tables share the CreateDate and ModifiedDate audit columns. We want to create a model that factors these common fields into a base entity so that we can simplify tracking the audit information.
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Figure 15-9. Client and Invoice tables share the CreateDate and ModifiedDate audit columns To create the model, do the following: 1. Add a new ADO.NET Entity Data Model to your project and import the Client and Invoice tables. Or update an existing model with these tables.
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Right-click the design surface and select Add Entity. Name the new entity Audit and check the Create key property box to create an entity key with the Id property. Move the audit fields CreateDate and ModifiedDate from the Client entity to the Audit entity. You can use Cut/Paste to move these. Delete these audit fields from the Invoice entity. Right-click the Audit entity and select Add Inheritance. Select the Audit entity as the base entity and the Client entity as the derived entity. Repeat Step 4, selecting Audit as the base entity and Invoice as the derived entity. Remove the ClientId from the Client entity. Remove the InvoiceId properties from the Invoice entity. We will use the Id property in the Audit entity as the entity key. Right-click the Audit entity and select properties. Change the Abstract property to True. This marks the Audit entity as abstract. Right-click the association between the Client entity and the Invoice entity. View the Properties of the association. Click the box in the Referential Constraints. Set the Principal as Client. Set the Dependent Property to the ClientId property. The dialog box should look like the one in Figure 15-10. Right-click the .edmx file in the Solution Explorer and select Open With XML Editor. Edit the entity set mapping in the Mapping layer as shown in the highlighted parts in Listing 15-11.
4. 5. 6.
7. 8.
Figure 15-10. The Referential Constraint for the Foreign Key association between the Client and Invoice entities Listing 15-11. Entity set mappings for the Audits <EntitySetMapping Name="Audits"> <EntityTypeMapping TypeName="IsTypeOf(EFRecipesModel.Client)">
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<MappingFragment StoreEntitySet="Client"> <ScalarProperty Name="Id" ColumnName="ClientId" /> <ScalarProperty Name="Name" ColumnName="Name" /> <ScalarProperty Name="CreateDate" ColumnName="CreateDate" /> <ScalarProperty Name="ModifiedDate" ColumnName="ModifiedDate" /> </MappingFragment> </EntityTypeMapping> <EntityTypeMapping TypeName="IsTypeOf(EFRecipesModel.Invoice)"> <MappingFragment StoreEntitySet="Invoice"> <ScalarProperty Name="Id" ColumnName="InvoiceId" /> <ScalarProperty Name="ClientId" ColumnName="ClientId" /> <ScalarProperty Name="Amount" ColumnName="Amount" /> <ScalarProperty Name="CreateDate" ColumnName="CreateDate" /> <ScalarProperty Name="ModifiedDate" ColumnName="ModifiedDate" /> </MappingFragment> </EntityTypeMapping> </EntitySetMapping> If the ClientId and InvoiceId columns, which alternately serve as the entity key for the Audit entity, do not overlap, the model is complete. The conflict here is that both the Client and Invoice entities are part of the Audits entity set. The entity key must be unique across this entity set. The entity key comes from both the ClientId column and the InvoiceId column. If they overlap, it is possible we would have non-unique keys in the Audits entity set. If these keys are naturally unique because they are drawn from two non-overlapping sets, then we are safe. This would be the case if, for example, both are identity columns with Client enumerating odd values (seed of 1, increment of 2) and Invoice enumerating even values (seed of 2, increment of 2). If ClientId and InvoiceId could conflict, then we need to remove the Audits entity set and move the Client and Invoice entities to their own entity sets. This involves converting the model so that the Audit entity is in both of the new entity sets. The use of multiple entity sets per type, better known as Multiple Entity Sets per Type (MEST), is not supported in the current designer. Converting the model to use MEST will prevent the designer from opening the model. To move the Client and Invoice entities to their own entity sets and represent the Audit entity in these two sets using MEST, continue with these steps: 10. Right-click the .edmx file and select Open With XML Editor. In the conceptual layer, replace the Audits definition with the XML in Listing 15-12. 11. Removing the Audits broke the association in our model. In the conceptual layer, replace the AssociationSet definition with the code in Listing 15-13. 12. Update the mapping layer with by replacing the entity set mapping for the Audits with the code in Listing 15-14. Listing 15-12. Replace the <EntitySet Name= Audits > tag with this code <EntitySet Name="Clients" EntityType="EFRecipesModel.Client" /> <EntitySet Name="Invoices" EntityType="EFRecipesModel.Invoice" /> Listing 15-13. Replace the <AssociationSet Name= FK_Invoice_Client > code with this code <AssociationSet Name="FK_Invoice_Client" Association="EFRecipesModel.FK_Invoice_Client"> <End Role="Client" EntitySet="Clients" />
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