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You have completed Entity Framework with ASP.NET MVC!
You have completed Entity Framework with ASP.NET MVC!
Preview
Before we see how to update the comic books controller to handle deletes, let's review my solution for implementing the `ValidateComicBook` method.
Follow Along
To follow along committing your changes to this course, you'll need to fork the dotnet-comic-book-library-manager repo. Then you can clone, commit, and push your changes to your fork like this:
git clone <your-fork>
cd dotnet-comic-book-library-manager
git checkout tags/v2.6 -b handling-deletes
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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
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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.
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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 =>
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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
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