MSDN Magazine - August 2008 - (Page 57)

Figure 8 Using Expand to Eagerly Load Related Entities public IList GetProducts(string productName, Categories category) { int categoryId = category.CategoryID; string uri = serviceUri + "/Categories(" + categoryId + ")/Products?"; if (!String.IsNullOrEmpty(productName)) { uri = uri + "$filter=ProductName eq '" + productName + "'&"; } uri = uri + "$expand=Categories"; Uri queryUri = new Uri(uri); try { IEnumerable products = context.Execute (queryUri); return products.ToList(); } catch (Exception) { return null; } To delete an entity instance, the object must again be held by the client and tracked by the DataServiceContext. Once tracked, we can make a simple call to the DeleteObject method on the context instance to mark the object for deletion: public string DeleteProduct(Products product) { context.AttachTo("Products", product); context.DeleteObject(product); context.SaveChanges(); } In all of the above examples, the last method call made on the context has been a call to the SaveChanges method. Changes are tracked in the DataServiceContext instance but not sent to the server immediately. Once all required changes for a given activity are complete, a call to SaveChanges will submit the changes to the data service. Batching } to PUT, and Delete mapped to DELETE. Just as with queries, the client library used (in this case, the .NET client library) will abstract away the details and use the appropriate HTTP verb, allowing the developer to use the provided APIs quite easily. To create an instance of an entity in the data service, simply create the .NET object, populate it with the desired data, and then call AddObject on the DataServiceContext object, passing the new object and the name of the EntitySet that the object will be added to, as seen here: public void AddProduct(Products product) { context.AddObject("Products", product); context.AttachTo("Categories", product.Categories); context.SetLink(product, "Categories", product.Categories); DataServiceResponse r = context.SaveChanges(); } So far, each of the examples has resulted in a single HTTP and sever request for each client operation. This approach (single operation per HTTP request) works well in some scenarios, but it is often a better idea to batch a number of operations and send them to the data service in a single HTTP request. This will reduce the number of round-trips to the data service and enable a logical scope of atomicity for collections of operations. To support these requirements, the client supports sending groups of CUD (Create, Update, Delete) operations and groups of Query operations to the data service in a single HTTP request. The following code shows how two or more queries can be sent to the data service as a single batch: var q = (DataServiceRequest)from o in context.SalesOrder select o; // send two queries in one batch request to the data service DataServiceResponse r = service.ExecuteBatch( new DataServiceRequest (new Uri("http://localhost:25115/NorthwindService.svc/Customers")), q); It’s also possible to add or change associations between .NET objects and have the client library reflect those changes as association creation/deletion operations. For example, the previous code creates a Product in the Northwind database and associates it with an existing Category. Categories and products participate in a one-tomany association; so, a given product has one specific category. After an entity is created, the data service will return a fresh copy of it, including values that have been updated as a result of triggers in the database, autogenerated keys, and so on. The client library will then automatically update the .NET object with the new values. To modify an existing entity instance, the object first needs to be fetched by the client and tracked by the DataServiceContext. The developer will first query for or attach the object to the context, make the desired changes to its properties, and then call the UpdateObject method. The UpdateObject method instructs the client library to send an update for that object: public void UpdateProduct(Products product) { Categories newCategory = product.Categories; context.AttachTo("Products", product); context.AttachTo("Categories", newCategory); context.UpdateObject(product); context.SetLink(product, "Categories", newCategory); } context.SaveChanges(); Sending a group of CUD operations as a batch to the data service is accomplished using a simple call to the SaveChanges method and passing SaveChangesOptions.Batch as the only parameter. This parameter value instructs the client to batch all the pending change operations into a batch and send them to the data service as an atomic group. When sending change operations as a batch using SaveChangesOptions.Batch, either all changes will complete successfully or none of the changes will be applied. Although this article covers just enough to get you started with ADO.NET Data Services, there are still a number of topics that you may be interested in. For more information, go to the Data Services Team blog at blogs.msdn.com/astoriateam and on the MSDN Data Development center at msdn.microsoft.com/data under the topic heading ADO.NET Data Services.  Elisa Flasko is a Program Manager in the Data Programmability team at MiMikE Flasko is a Program Manager in the SQL Data Programmability group at Microsoft. You can contact Mike through his blog at blogs.msdn.com/mflasko. August 2008 57 Learning More crosoft, including the ADO.NET technologies, XML technologies, and SQL Server Connectivity technologies. She can be reached at blogs.msdn.com/elisaj. msdnmagazine.com http://blogs.msdn.com/astoriateam http://msdn.microsoft.com/data http://blogs.msdn.com/elisaj http://blogs.msdn.com/mflasko http://msdnmagazine.com

Table of Contents Feed for the Digital Edition of MSDN Magazine - August 2008

MSDN Magazine - August 2008 Toolbox CLR Inside Out Basic Instincts Cutting Edge Patterns in Practice Data 2.0 - Expose And Consume Data In A Web Services World Biztalk EDI - Build A Robust EDI Solution With BizTalk Server Silverlight - Create Animations With XAML And Expression Blend Write On! - Create Web Apps You Can Draw On With Silverlight 2 Wicked Code - Craft Custom Controls For Silverlight 2 Team System Foundations Windows With C++ Concurrent Affairs Going Places { End Bracket }

MSDN Magazine - August 2008

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