Handling Deletes

Before we see how to update the Comic Books Controller to handle deletes, 0:00 let's review my solution for implementing the ValidateComicBook method. 0:04 To review, the ValidateComicBook method, which is called from both the add and 0:08 edit post action methods, needs to be updated with a query 0:13 that checks if the provided issue number is already in use or not. 0:17 To do that, we need to write a query that checks if the database contains 0:21 a different Comic Book that is associated with the same Series, and 0:26 has the same Issue Number. 0:30 Whenever I need to check for the existence of a record or 0:32 entity in the database without actually returning the entity, 0:35 itself, I typically use the Any link operator, 0:38 which will return true if an entity is found that matches our provided criteria. 0:42 For the predicate, I'm looking for a comic book whose ID property value 0:46 doesn't equal the Passed In comic book;s ID property value. 0:51 But whose Series ID and 0:55 Issue Number properties are equal to the Passed In Comic Book's property values. 0:56 This query will return true if a comic book other than the Passed In comic book 1:01 is found that it's associated with the same series and has the same issue number. 1:06 Let's test the validation method and see if it behaves as expected. 1:11 Let's try adding another Comic Book for 1:15 the Series, Bone with an Issue Number of four. 1:18 And select Jeff Smith for the Artist and Script for the Role. 1:25 Click the Save button and we get an error message. 1:31 The provided Issue Number has already been entered for the selected Series. 1:36 Okay, so far, so good. 1:41 Let's change the Issue Number to five and try saving again. 1:43 And this time, the Comic Book was successfully added. 1:47 Now, let's make sure that we can edit the Comic Book without 1:51 getting a false positive or an existing Issue Number. 1:53 Set an Average Rating value and click the Save button. 1:59 And here's our updated Comic Book. 2:03 Great, now, we have Comic Book Creates and 2:06 Updates, including the server side validation fully implemented. 2:08 Now, let's turn our attention to implementing the letter d in crud deletes. 2:13 There are two action methods that we need to update. 2:18 The Delete Get action method And 2:21 the Delete Post action method. 2:26 The Delete Get action methods only parameter is a nullable 2:33 integer-named id like the detail and adequate action methods. 2:37 If the id parameter is null, 2:41 we immediately return a BadRequest HttpStatusCode. 2:43 Otherwise, we attempt to retrieve the comicBook from 2:49 the database with the related Series eagerly loaded. 2:52 And we return a NotFound HTTP Status Code if we don't find a comicBook. 2:56 Then, we call the View method, passing in the comicBook instance. 3:02 The query to retrieve the comic book can use the link, Where operator 3:06 in order to retrieve the comicBook with a Passed In ID parameter value, 3:10 followed by a call to the SingleOrDefault method to execute the query. 3:14 Don't forget the call to the Include operator in order to eagerly load 3:18 the related Series. 3:23 _context.ComicBooks.Include(cb => 3:24 cb.Series).Where(cb => 3:32 cb.Id == id).SingleOrDefault(). 3:38 The Delete Post action method is pretty straight forward. 3:47 It accepts an id parameter value which is used to delete the comicBook. 3:50 Then, we set the message key in the TempData dictionary and 3:54 re-direct the user to the Index action, which is our web apps home page. 3:58 To delete the comic book from the database, 4:03 we can create a comic book entity using the id parameter value that we're passed. 4:05 Attach it to the context using the entry method. 4:11 And set its entry state to deleted. 4:14 And call the context's save changes method in order to perform the deletion in 4:18 the database. 4:23 var comicBook = new ComicBook(). 4:24 And set { Id = id};. 4:30 Then, _context.Entry (comicBook).State 4:33 = EntityState.Deleted. 4:40 And finally, _context.SaveChanges();. 4:44 Let's test our changes To delete a comic book, you 4:50 first need to view the Comic Book Detail page of the comic book you want to delete. 4:57 I'll click the Detail button on Issue Number five of the Bone Series. 5:03 Then, on the Comic Book Detail page, click the Delete button. 5:07 We're presented with Delete Comic Book page to confirm the deletion. 5:12 You can click the Cancel button to return to the Detail page, but I'll go ahead and 5:16 click the Delete button to delete the comic book. 5:20 We're then redirected to the Comic Books page 5:24 which displays a message confirming the comic book deletion. 5:27 We can also see that Issue Number 5 of the Bone Series 5:31 is no longer in the list of Comic Books. 5:34 With those changes completed, we can now create, update, and delete comic books. 5:37 Nice work making progress on our web app. 5:42 It's worth noting that our approach for deleting a comic book 5:45 relies upon cascade deletes to delete the related comic book artist records. 5:48 This works fine for comic books, but for artists or series, we may not want 5:52 users to be able to delete a large number of related comic book records. 5:57 Let's look at a couple of alternative strategies for handling deletes. 6:02 One option would be to retrieve and display related data 6:05 on the detail page for a record that the user is trying to delete. 6:09 For instance, under Artist Detail page, 6:12 we would display all of the comic books that the artist is associated with. 6:15 We could then disable the Delete button, and 6:19 display a tool tip that lets the user know that the artist must first be removed from 6:21 all of the listed comic books before the artist can be deleted. 6:25 This is much more of a conservative approach than just 6:29 letting the cascade deletes happen. 6:32 But while it prevents the deletion of records that are in use, 6:34 it does give the user the information that they need in order to move forward. 6:37 If they, in fact, need to actually delete the record in question, another option 6:41 would be to allow the user to delete the records, even if they are in use. 6:46 But instead of physically deleting the record from the database, 6:50 we simply mark the record as deleted. 6:53 This is typically done by setting an active property to the value of false. 6:55 These kinds of deletes are known as Logical Deletes. 7:00 Allowing users to perform Logical Deletes 7:03 give developers a way to grant users more power in the application 7:06 while still retaining some level of control over the data. 7:10 For example, a user might delete a record, 7:13 only to realize later that they shouldn't have done that. 7:16 To undo the deletion, they could contact an administrator or 7:20 superuser do it in turn. 7:24 Use a special page to search for and undeleted the deleted record. 7:26 By saving this active property back to a value of true. 7:30 Logical Deletes seem too good to be true, right? 7:32 Well, there are some draw backs to be aware of. 7:37 If your web app requires users to delete large numbers of records, 7:39 your database can grow unexpectantly large, 7:43 as deleted records are never actually removed from the database. 7:46 Another draw back is that most of your queries will need to filter 7:50 out records who's active values are false. 7:54 Otherwise, list pages in the web app might unintentionally contain deleted records. 7:57 None of these issues are insurmountable. 8:02 There are simply things you need to consider when you're deciding 8:04 how you handle deletes in your web app. 8:07 Furthermore, you can even mix and match approaches within the same app. 8:09 Critical record types could use logical deletes, 8:14 while other record types could use physical deletes. 8:17 Coming up next, we'll review the Comic Book Artist Controller, and 8:20 give you a chance to finish implementing it on your own. 8:24