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Before we see how to update the Comic
Books Controller to handle deletes,

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let's review my solution for
implementing the ValidateComicBook method.

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To review, the ValidateComicBook method,
which is called from both the add and

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edit post action methods,
needs to be updated with a query

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that checks if the provided issue
number is already in use or not.

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To do that, we need to write a query
that checks if the database contains

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a different Comic Book that is
associated with the same Series, and

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has the same Issue Number.

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Whenever I need to check for
the existence of a record or

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entity in the database without
actually returning the entity,

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itself, I typically use
the Any link operator,

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which will return true if an entity is
found that matches our provided criteria.

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For the predicate, I'm looking for
a comic book whose ID property value

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doesn't equal the Passed In
comic book;s ID property value.

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But whose Series ID and

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Issue Number properties are equal to the
Passed In Comic Book's property values.

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This query will return true if a comic
book other than the Passed In comic book

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is found that it's associated with the
same series and has the same issue number.

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Let's test the validation method and
see if it behaves as expected.

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Let's try adding another Comic Book for

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the Series,
Bone with an Issue Number of four.

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And select Jeff Smith for
the Artist and Script for the Role.

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Click the Save button and
we get an error message.

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The provided Issue Number has already
been entered for the selected Series.

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Okay, so far, so good.

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Let's change the Issue Number to five and
try saving again.

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And this time,
the Comic Book was successfully added.

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Now, let's make sure that we
can edit the Comic Book without

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getting a false positive or
an existing Issue Number.

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Set an Average Rating value and
click the Save button.

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And here's our updated Comic Book.

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Great, now, we have Comic Book Creates and

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Updates, including the server side
validation fully implemented.

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Now, let's turn our attention to
implementing the letter d in crud deletes.

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There are two action methods
that we need to update.

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The Delete Get action method And

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the Delete Post action method.

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The Delete Get action methods only parameter is a nullable

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integer-named id like the detail and
adequate action methods.

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If the id parameter is null,

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we immediately return
a BadRequest HttpStatusCode.

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Otherwise, we attempt to
retrieve the comicBook from

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the database with the related
Series eagerly loaded.

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And we return a NotFound HTTP Status Code
if we don't find a comicBook.

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Then, we call the View method,
passing in the comicBook instance.

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The query to retrieve the comic book
can use the link, Where operator

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in order to retrieve the comicBook
with a Passed In ID parameter value,

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followed by a call to the SingleOrDefault
method to execute the query.

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Don't forget the call to the Include
operator in order to eagerly load

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the related Series.

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_context.ComicBooks.Include(cb =>

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cb.Series).Where(cb =>

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cb.Id == id).SingleOrDefault().

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The Delete Post action method
is pretty straight forward.

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It accepts an id parameter value which
is used to delete the comicBook.

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Then, we set the message key
in the TempData dictionary and

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re-direct the user to the Index action,
which is our web apps home page.

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To delete the comic
book from the database,

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we can create a comic book entity using
the id parameter value that we're passed.

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Attach it to the context
using the entry method.

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And set its entry state to deleted.

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And call the context's save changes method
in order to perform the deletion in

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the database.

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var comicBook = new ComicBook().

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And set { Id = id};.

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Then, _context.Entry (comicBook).State

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= EntityState.Deleted.

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And finally, _context.SaveChanges();.

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Let's test our changes To
delete a comic book, you

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first need to view the Comic Book Detail
page of the comic book you want to delete.

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I'll click the Detail button on
Issue Number five of the Bone Series.

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Then, on the Comic Book Detail page,
click the Delete button.

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We're presented with Delete Comic Book
page to confirm the deletion.

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You can click the Cancel button to return
to the Detail page, but I'll go ahead and

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click the Delete button
to delete the comic book.

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We're then redirected
to the Comic Books page

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which displays a message confirming
the comic book deletion.

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We can also see that Issue Number
5 of the Bone Series

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is no longer in the list of Comic Books.

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With those changes completed, we can now
create, update, and delete comic books.

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Nice work making progress on our web app.

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It's worth noting that our approach for deleting a comic book

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relies upon cascade deletes to delete
the related comic book artist records.

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This works fine for comic books, but
for artists or series, we may not want

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users to be able to delete a large
number of related comic book records.

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Let's look at a couple of alternative
strategies for handling deletes.

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One option would be to retrieve and
display related data

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on the detail page for a record
that the user is trying to delete.

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For instance, under Artist Detail page,

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we would display all of the comic books
that the artist is associated with.

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We could then disable the Delete button,
and

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display a tool tip that lets the user know
that the artist must first be removed from

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all of the listed comic books
before the artist can be deleted.

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This is much more of
a conservative approach than just

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letting the cascade deletes happen.

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But while it prevents the deletion
of records that are in use,

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it does give the user the information
that they need in order to move forward.

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If they, in fact, need to actually delete
the record in question, another option

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would be to allow the user to delete
the records, even if they are in use.

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But instead of physically deleting
the record from the database,

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we simply mark the record as deleted.

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This is typically done by setting
an active property to the value of false.

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These kinds of deletes
are known as Logical Deletes.

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Allowing users to perform Logical Deletes

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give developers a way to grant
users more power in the application

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while still retaining some
level of control over the data.

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For example, a user might delete a record,

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only to realize later that
they shouldn't have done that.

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To undo the deletion,
they could contact an administrator or

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superuser do it in turn.

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Use a special page to search for
and undeleted the deleted record.

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By saving this active property
back to a value of true.

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Logical Deletes seem too good to be true,
right?

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Well, there are some draw
backs to be aware of.

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If your web app requires users to
delete large numbers of records,

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your database can grow
unexpectantly large,

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as deleted records are never
actually removed from the database.

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Another draw back is that most of
your queries will need to filter

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out records who's active values are false.

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Otherwise, list pages in the web app might
unintentionally contain deleted records.

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None of these issues are insurmountable.

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There are simply things you need
to consider when you're deciding

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how you handle deletes in your web app.

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Furthermore, you can even mix and
match approaches within the same app.

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Critical record types
could use logical deletes,

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while other record types
could use physical deletes.

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Coming up next, we'll review
the Comic Book Artist Controller, and

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give you a chance to finish
implementing it on your own.