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Now that we've gone through the process
of adding repositories to our web app,

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let's talk about the pros and
cons of the repository design pattern.

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Moving all of our data retrieval and
persistence code into a repository.

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Or in our case, a collection of
repositories helps to simplify and

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keep our controllers as lean as possible.

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It also helps us to adhere to the dry or
don't repeat yourself design principle.

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Instead of duplicating EF queries or
commands across controller action methods,

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we can place that code in
a repository method and

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call that method from our
action methods instead.

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But adding repositories to an application
adds an additional layer of abstraction

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between your controller action methods and
the database context.

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And in some cases this
layer of abstraction can

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feel like you're adding complexity
without enough of a clear benefit.

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As an alternative to using repositories,
we can use Query and Command classes.

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Query and Command classes encapsulate
data persistence code much in

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the same way that repositories do.

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But when you selectively on an as needed
basis they can provide the same benefits

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repositories without
adding as much complexity.

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Let's look at a couple of examples.

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Before we add our first class let's add
two folders to the shared class library

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data folder one named queries and
another named commands.

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As the folder name suggests we'll add all
of our query classes to the queries folder

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and all of our command classes
to the commands folder.

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To make it easier to compare using
repositories to using queries and commands

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I added another copy of the comic book
artist controller class to our project.

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And reverted the code back to what it was

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before we added our comic
book artist repository.

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If you're following along you can find a
copy of this code in the teacher's notes.

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Let's start with
the Add.getAction() method.

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At the beginning of the method we have
a query to retrieve the comic book

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that the user is adding an artist to.

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When using query in command
classes you don't automatically

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move every query in
command into it's class,

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instead you evaluate it's
relative complexity and usage.

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And make a determination if your code and

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overall design will benefit from moving
the query or command to it's own class.

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This query isn't overly complex but
it's used at least twice.

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Once here in the ad get action method and
once in the ad post action method.

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Since it's duplicated and
it's what I'd call moderately complex,

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let's move it to a query class.

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Add a class named get comic book
query to the queries folder.

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To start, we need a private field for
the context.

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And a constructor that accepts a context
instance and sets the private field.

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Private Context _context = null.

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Public GetComicBookQuery then
a parameter up type Context

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named context and set the private
field to that parameter value.

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Then let's add a method name execute
that returns a comic book instance.

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And accepts an integer ID value, public.

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ComicBook Execute, int id.

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For the implementation we can switch
back to the ControllerAction method.

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And copy the query to the clipboard.

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Add return and
then paste the query from the clipboard.

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We've got some things to fix up here.

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Change context to, _context.

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ComicBookId to just id.

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And we need to add using directive for
the system data

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entity namespace,
using System.Data Entity.

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Now we're ready to update the action
method to use our new query class.

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Remove the existing query, instantiate an
instance of the GetComicBookQuery class,

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new GetComicBookQuery.

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Add a using directive for
the ComicBookSharedDataQueries

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namespace passing the context property
from the base controller class.

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And call the execute method, passing
in the comicBookId parameter value.

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Then in the add post action method we
can replace the query to set the view

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viewModel's ComicBook property with
another instance of our query class.

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Now let's review the remaining EF queries
in commands that are being used in this

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controller.

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And determine if any of them
would be good candidates for

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moving to their own query or
command classes.

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Remember we only want to create query or

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command classes to encapsulate complex or
duplicated code.

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In the add post action method we've
got some code related to add in

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a comic book artist.

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This code isn't repeated anywhere else and
its relatively simple.

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So let's go ahead and
just leave it as it is.

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In the delete get action method we've got
a query to retrieve a comic book artist.

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This query is about the same level
of complexity as the query that we

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just moved to a query class.

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But for
now it's only used in this one location.

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Again for
simplicity's sake let's leave it as it is.

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In the delete post action method we've got

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code related to deleting
a comic book artist.

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But again it's relatively simple and
it's not repeated anywhere else.

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So let's leave it alone.

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In the validate ComicBookArtists method
we've got a query that checks if

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the provided artist and

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role combination has already been
added to the comic book or not.

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Again, just like the other queries or
commands that we just looked at

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it's only moderately complex, and
it only appears in this one location.

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So let's just leave it alone.

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Because it's query or
command, it's its own class.

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You might expect that you'd end up with
a lot more classes in your project,

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than if you had used repositories.

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But as you just saw the number of
classes that you'll end up with

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really depends on the complexity
of your apps queries and commands.

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And whether or not they're used for
more than one location.

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Even if you end up with more classes
in your project than if you had used

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repositories, using query and command
classes often make it a little easier

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to find a query or
command that you're looking for.

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Since each class contains
a single query or command.

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You don't have to determine which
repository you're looking for

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then find the specific method
within that repository.

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You just find the query command
that you're looking for and

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directly open that file.

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Let's go ahead and

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convert the command to add a ComicBooks
artist to use the command class.

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So we can look at an example.

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At a class named AddComicBookArtistCommand
to the Commands folder.

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As we've see before we need
a private field for the context and

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a constructor that accepts a context
instance, and sets the private field.

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Private Context_context = null; public

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AddComicBookArtistCommand then a parameter
of type context named context.

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And set the private field
o the parameter value.

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Then add a method named
execute that returns void.

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And accepts three integer values

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comicBookId, artistsId and roleId.

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For the implementation we can grab a copy
of the code in the controller action

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method.

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And copy the code to add a comic
book artist to the clipboard.

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Paste the code from the clipboard.

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And probably to no one's surprise
got some things to fix up again.

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Add a using directive for

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the comic book shared model's name space. Remove the bad references to view model

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and lower case these
identifiers ComicBookID,

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ArtistId and roleId.

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Whoops, looks like I have
misspelled this parameter.

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ArtistId not artistsId,
then replace Context with _context.

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Now we're ready to update the action
method to use our new command class.

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Remove the existing code.

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Instantiate an instance of
the AddComicBookArtistCommand class.

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Add a using directive for the missing name
space; ComicBookShared.Data.Commands.

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Pass in the context property from
the base controller class and

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call the execute method.

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Passing in the viewModel.ComicBookId,
artistId.

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Enroll Id property values.

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Now we're ready to test our changes.

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To do that we need to comment out
the ComicBooks artist controller class.

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And rename the copy of the controller that
we've been modifying to the correct name.

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So that ASP.NET MVC's routing engine will find the query and

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command version of the controller.

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Change the name of
the class is good enough.

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We can leave the file name alone.

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Let's separate points in the two methods
that we updated to use query and

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command classes just to make sure that
our testing exercises, those code pass.

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And run the WebApp.

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View the details for Bone #3,
and add a new artist.

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And we've hit our first break point.

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Press F5 to continue execution.

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Select Jeff Smith for the artist.

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Pencils.

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Oop, that artist is already in use.

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So choose Archie Goodwin Pencils and save.

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And we're at our second breakpoint.

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Looks like we're good to go.

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Determining how best to organize your data
access code can be a challenging exercise.

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That being said there's no need
to agonize too much about it.

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Consider the requirements of your app,
weigh the pros and cons of each approach.

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And choose the one you and
your team thinks will be the best fit.

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Just remember to not
overbuild your solution,

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only build the features
that you actually need.

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You could always refactor later on.

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Let's do a quick review of the section.

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We created a base controller class in
order to eliminate code duplication in our

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controllers.

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We also reviewed
the repository design pattern.

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And implemented a new repository for
our web app.

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And we saw how to break
apart that repository

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into entity type focused repositories and

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created a generic base repository class
to further reduce code duplication.

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Then we finished up with
a look at how query and

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command classes could be used as
an alternative to repositories.

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Next up, we'll finish the Comic Book
Library Manager Web App by updating

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the series and artists sections to use EF.

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See you after the break.