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this.context.SaveChanges(); return customer; } } Listing 9-23. We extend the EFRecipesEntities class with this partial class public partial class EFRecipesEntities { public void StartSelfTracking() { var entities = this.ObjectStateManager .GetObjectStateEntries(~EntityState.Detached) .Where(e => !e.IsRelationship) .Select(e => e.Entity) .OfType<IObjectWithChangeTracker>(); foreach (var entity in entities) { entity.StartTracking(); } } public override int SaveChanges(SaveOptions options) { var affected = base.SaveChanges(options); if (SaveOptions.AcceptAllChangesAfterSave == (SaveOptions.AcceptAllChangesAfterSave & options)) { var entities = this.ObjectStateManager .GetObjectStateEntries(EntityState.Unchanged) .Where(e => !e.IsRelationship) .Select(e => e.Entity) .OfType<IObjectWithChangeTracker>(); foreach (var entity in entities) { entity.AcceptChanges(); } } return affected; } } Listing 9-24. Our ASP.NET page demonstrating reading, creating, and updating a customer <body> <form id="form1" runat="server"> <div> <asp:Button ID="button1" Text="Create Customer" OnClick="CreateCustomer" runat="server" /> <br /> <asp:Button ID="button2" Text="Read Customer" OnClick="ReadCustomer" runat="server" />
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<br /> <asp:Button ID="button3" Text="Update Customer" OnClick="UpdateCustomer" runat="server" /> <br /> <h2>Customer Details</h2> <table> <tr> <td>Customer Name</td> <td><asp:Label ID="CustomerName" runat="server" /></td> </tr> <tr> <td>Phone Number</td> <td><asp:Label ID="PhoneNumber" runat="server" /></td> </tr> </table> </div> </form> </body> Listing 9-25. The code behind for our ASP.NET page public partial class Default : System.Web.UI.Page { protected void Page_Load(object sender, EventArgs e) { if (!this.IsPostBack) { using (var context = new EFRecipesEntities()) { // delete previous test data context.ExecuteStoreCommand("delete from chapter9.phone"); context.ExecuteStoreCommand("delete from chapter9.customer"); } } } private void ShowCustomer() { if (this.Session["Customer"] != null) { Customer customer = (Customer)this.Session["Customer"]; this.CustomerName.Text = customer.Name; if (customer.Phones.Count > 0) { this.PhoneNumber.Text = customer.Phones[0].Number; } } } protected void CreateCustomer(object sender, EventArgs e) { var customer = new Customer { Name = "Phillip Marlowe", Company = "Chandler Enterprises" };
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customer.Phones.Add(new Phone { Number = "212 555-5555", PhoneType = "Office" }); using (var repository = new CustomerRepository()) { repository.SubmitCustomerWithPhones(customer); } } protected void ReadCustomer(object sender, EventArgs e) { using (var repository = new CustomerRepository()) { this.Session["Customer"] = repository.GetCustomer("Phillip Marlowe"); } ShowCustomer(); } protected void UpdateCustomer(object sender, EventArgs e) { Customer customer = (Customer)this.Session["Customer"]; var number = customer.Phones .FirstOrDefault(p => p.PhoneType == "Office"); if (number != null) number.MarkAsDeleted(); customer.Phones.Add(new Phone { Number = "817 867-5309", PhoneType = "Cell" }); using (var repository = new CustomerRepository()) { var cust = repository.SubmitCustomerWithPhones(customer); cust.StartTracking(); this.Session["Customer"] = cust; } ShowCustomer(); } } To test the code, click the Create Customer button. Nothing interesting is changed in the browser, but this causes our test data to be inserted into the database. Once the test data is inserted, click the Read Customer button. This reads the customer entity and displays it on the page (see Figure 9-10). Finally, click the Update Customer button. This changes the customer s phone number and displays the updated customer and phone number (see Figure 9-11).
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Figure 9-10. The web page after the customer is created and the customer is read and stored in the session state
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Figure 9-11. The web page after the customer has been updated and the new updated customer has been inserted into the session state
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How It Works
Self-tracking entities make n-tier development easier because they implement their own change tracking mechanism. Self-tracking entities record changes without the support of an object context. When sent to the server, these changes can be replayed in the presence of an object context using the ApplyChanges() method. To make this process easier in WCF scenarios, the generated code for self-tracking entities hooks into the deserialization process using the OnDeserializedAttribute. This code turns on change tracking before the entity is received by the client. This makes change tracking with self-tracking entities fairly seamless in a WCF environment. The story is a little more manual outside of WCF. In this recipe, we used the self-tracking entities in an ASP.NET environment. To turn on change tracking, we implemented the StartSelfTracking() method. In this method we find all the self-tracking entities in the object state manager and for each call StartTracking(). This sets the ChangeTracker.EnableChangeTracking property to true and initiates change tracking for the entity. Change tracking is also enabled on an entity in two other cases. It is enabled when the entity is attached to another entity that has change tracking enabled. It is also enabled when the state of the selftracking entity is changed with MarkAsUnchanged(), MarkAsModified(), or MarkAsDeleted(). Change tracking is disabled or stopped on a self-tracking entity when you call StopTracking() or after you apply the changes on the entity to object context using ApplyChanges(). The reason why the internal mechanism for change tracking stops in a self-tracking entity when ApplyChanges() is called is that at this point the entity is attached to the object context. The object context is now responsible for tracking changes. As with any other POCO entity, after changes have been made you can call DetectChanges() to cause snapshot-based change tracking or you can simply call SaveChanges() which internally calls DetectChanges(). For other entities, SaveChanges() changes the state of each entity to Unchanged. However, for selftracking entities, SaveChanges() leaves the state as Added. In Listing 9-23 we override the SaveChanges() method to manually change the state of the self-tracking entities (those that implement IObjectWithChangeTracker). When the Create Customer button is clicked, the code behind (Listing 9-25) creates our test customer entity and calls SubmitCustomerWithPhones() to insert the new customer and the phone numbers for the customer. When a new customer entity is created, change tracking is disabled. We don t need to turn it on at this point because the entity is in the Added state and we have no changes to apply. When the Read Customer button is clicked, the customer is read from the repository using the GetCustomer() method. This method turns on change tracking for the entities by calling our StartSelfTracking() method (see Listing 9-23). When the Update Customer button is clicked, the code behind reads the customer from the session state, makes some changes to the customer s phones, and submits these changes using the CustomerRepository s SubmitCustomerWithPhones() method. The customer entity that is returned has change tracking disabled. We explicitly enable this with StartTracking() before we assign the customer back to the session state. You might be tempted to use the ObjectMaterialized event to turn on tracking for all entities when they are materialized. This is not a viable approach because the ObjectMaterialized event only guarantees that regular properties are initialized before the event is raised. Collection properties might change after the event has been raised. If you turn on change tracking while handling the ObjectMaterialized event, you may start tracking changes on collections that are simply part of the normal object materialization process.
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