barcodelib c# CUSTOMIZING ENTITY FRAMEWORK OBJECTS in Font

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CUSTOMIZING ENTITY FRAMEWORK OBJECTS
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{ public override int SaveChanges(SaveOptions options) { Console.WriteLine("Saving Changes..."); var applicants = this.ObjectStateManager .GetObjectStateEntries(EntityState.Deleted) .Select(e => e.Entity) .OfType<Applicant>().ToList(); int changes = base.SaveChanges(options); Console.WriteLine("\n{0} applicants deleted", applicants.Count().ToString()); foreach (var app in applicants) { File.Delete(app.ResumePath); Console.WriteLine("\n{0}'s resume at {1} deleted", app.Name, app.ResumePath); } return changes; } } The following is the output from the code in Listing 12-1: Saving Changes... 0 applicants deleted Saving Changes...
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1 applicants deleted
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Alex Jones's resume at AlexJones.txt deleted
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The code in Listing 12-1 starts by inserting two applicants, each with the path to a resume file that we also created. The goal here is to delete the resume file in a structured way when the instance of the Applicant entity is deleted. We do this by overriding the SaveChanges() method. In our SaveChanges() method, we first gather up all the instances of Applicant that have been marked for deletion. These are the ones that will be deleted from the database when we call the real SaveChanges() method. We need to get them before we call SaveChanges() because after we call SaveChanges(), these instances will be detached from the context and we will no longer be able to use this query to retrieve them. Once we have the instances that will be deleted, we call SaveChanges() to do
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the real work of persisting objects to the database. Once the changes have been successfully committed, we can delete the resume files. There are a couple of possible variations worth noting. We could wrap both our call to SaveChanges() and the file deletions in a transaction to ensure that both either succeed or fail together. Another variation is to use the DetectChangesBeforeSave SaveOption in our call to SaveChanges(). This option preserves the tracking information in the object context and would allow us to move the query for deleted objects to somewhere after the call the SaveChanges(). To clear the tracking information, we would need to call AcceptAllChanges() before we leave the method. Entity Framework does not expose insert, update, and delete events for each entity. However, much of what we would do in these events can be handled, as we have demonstrated here, by overriding the SaveChanges() method.
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12-2. Validating Property Changes
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Problem
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You want to validate a value being assigned to a property.
Solution
Let s say you have a model with a User entity. The User entity has properties for the full name and user name for the user. You have a business rule that says each user must have a UserName greater than five characters long. You want to enforce this business rule with code that sets the IsActive property to false if the UserName is set to a string less than or equal to five characters; otherwise the IsActive flag is set to true. The model is shown in Figure 12-2.
Figure 12-2. The User entity in our model To enforce our business rule, we need to implement the partial methods OnUserNameChanging() and OnUserNameChanged(). These methods are called during the property change activity and after the property has been changed. The code in Listing 12-2 demonstrates one solution. Listing 12-2. Monitoring the changing of the UserName property class Program { static void Main(string[] args)
CUSTOMIZING ENTITY FRAMEWORK OBJECTS
{ RunExample(); } static void RunExample() { using (var context = new EFRecipesEntities()) { var user1 = new User { FullName = "Robert Meyers", UserName = "RM" }; var user2 = new User { FullName = "Karen Kelley", UserName = "KKelley" }; context.Users.AddObject(user1); context.Users.AddObject(user2); context.SaveChanges(); Console.WriteLine("Users saved to database"); } using (var context = new EFRecipesEntities()) { Console.WriteLine(); Console.WriteLine("Reading users from database"); foreach (var user in context.Users) { Console.WriteLine("{0} is {1}, UserName is {2}", user.FullName, user.IsActive "Active" : "Inactive", user.UserName); } } } } public partial class User { partial void OnUserNameChanging(string value) { if (value.Length > 5) Console.WriteLine("{0}'s UserName changing to {1}, OK!", this.FullName, value); else Console.WriteLine("{0}'s UserName changing to {1}, Too Short!", this.FullName, value); } partial void OnUserNameChanged() { this.IsActive = (this.UserName.Length > 5); } } The following is the output of the code in Listing 12-2:
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