Handling Reads

Now that we have EF configured, 0:00 we're ready to update our first controller to read data from the database. 0:02 But before we do that, 0:06 let's talk about the lifetime of our database context object instances. 0:07 A context object's lifetime, for the sake of our discussion, 0:12 is the time from when it's instantiated to when it's disposed. 0:15 We don't want to instantiate a context every time that we need to retrieve or 0:19 persist data. 0:23 But we also don't want a context that lives for too long. 0:25 A long living context can, 0:28 over time, end up consuming more system memory than we would want. 0:29 And have a negative impact on the performance of our application. 0:34 So, what are our options? 0:37 Let's review how MVC handles requests. 0:40 The process begins with a user entering a URL into a browser's address bar, 0:42 which causes an HTTP request to be sent to the server. 0:47 The server receives the request and hands it off to the MVC Framework. 0:50 When processing the request, 0:55 MVC uses simple pattern matching against the URL path. 0:57 To determine the name of the controller and the action that should be used 1:00 to handle the request in the form of controller / action. 1:04 So the path ComicBooks / Index 1:09 would map to a controller named ComicBooks and an action method named Index. 1:12 This process is known as URL routing. 1:17 Once the controller and action method have been identified, 1:20 MVC then instantiates an instance of the controller class. 1:23 And calls the appropriate action method on that instance. 1:27 The action method returns an action result which in turn is rendered and 1:30 sent back to the user's browser as an HTTP response. 1:35 [SOUND] This process is repeated for each request. 1:39 Given that, each request is associated with its own instance of the appropriate 1:43 controller class. 1:47 Aligning the lifetime of our context with the controller's lifetime, 1:49 gives us a convenient way of managing the lifetime of the context. 1:53 Adding a default constructor to the Comic Book's controller class will allow us to 1:57 instantiate an instance to the context when the controller is instantiated. 2:01 That takes care of how to manage the start of the context's lifetime. 2:06 In the next video, we'll see how we can manage the end of the context's lifetime, 2:10 by disposing our context when the controller is disposed. 2:14 Let's jump back into the Comic Book's controller, and 2:18 make our first set of changes. 2:21 We need a private field to hold a reference to the context object instance. 2:23 Private Context, then press Ctrl period or 2:27 dot to add the using directive for the ComicBookShared.Data name space. 2:31 We'll start the private field name with an underscore, so 2:38 that it's easy to identify throughout our code. 2:41 _context, and initialize the field to null, 2:43 and a public default constructor 2:49 And initialize the private field to an instance at the context class. 2:59 Now that we have our context's instance, let's add our first query. 3:03 Starting with the index action method, we need a query that returns 3:07 a list of comic books with the related series, entities eagerly loaded. 3:11 We can grab the query that we need from the repository class GetComicBooks method. 3:16 Select the code for the query starting with the DB set property, and 3:21 copy to the clipboard. 3:25 Then in the index action method, 3:27 replace the comic books collection initialization with the reference 3:30 to the context private field followed by the contents in the clipboard. 3:33 Moving on to the detail action method, we need a query that returns a single comic 3:38 book with the related series and artist entities eagerly loaded. 3:43 Again, we can grab the query that we need from the repository class, 3:47 this time from the GetComicBook method. 3:51 Select the query and copy to the clipboard. 3:55 Then, in the detail action method, replace the comic book object initialization 3:57 with the reference to the context private field, 4:02 followed by the contents of the clipboard. 4:05 And change the comic book ID variable to just ID. 4:07 Which is the name of the parameter being passed to the action method. 4:17 And now the add get action method. 4:21 In this action method, we need to pass the context into the viewModel's Init method, 4:30 and that's easy to do. 4:35 Almost just as soon as we finish typing _context, 4:37 Visual Studio will let us know that we have a compilation error. 4:41 Now, we need to update the method definition 4:44 to accept an instance of the context class. 4:47 I'll put my cursor on the method name and 4:50 press F12 to browse to the method definition. 4:52 Add a parameter of type Context named context, and now we get a new error. 4:55 ComicBooksAddViewModel.Init context no suitable method found to override. 5:01 The viewModel class inherits from the ComicBooksBaseViewModel class, and 5:07 overrides its Init method. 5:12 When overriding a method in the base class, 5:14 the method signature needs to match the method that you're overriding. 5:16 To resolve this error, we need to update the Init method in the base class 5:20 to have the same signature as the Init method in the descendant class. 5:25 Browse through the base class method, 5:29 by placing the cursor on the call to that method, and pressing F12. 5:31 And add a parameter of type Context. 5:36 Named context. 5:41 While we're here, let's replace this new list of series entities 5:43 with a query to get the list of series from the context. 5:47 context.Series.OrderBy(s => 5:50 s.Title).ToList(),. 5:56 We can browse back the ComicBooksAddViewModel class by 6:02 pressing Ctrl minus twice. 6:06 Now we're getting an error with our call to the base class Init method. 6:08 There is no argument given that corresponds to the required formal 6:12 parameter context of ComicBooksBaseViewModel.Init(Context). 6:16 We need to pass the Context parameter to the Init method call. 6:21 And we can replace the new list of artists and 6:25 roles entities with queries to get those respective lists. 6:28 Context.Artists.OrderBy(a => 6:31 a.Name).ToList(), and for roles. 6:38 Context.Roles.OrderBy(r => 6:49 r.Name).ToList(),. 6:54 Let's press Ctrl+Shift+B to build our project and check the state of things. 6:58 We got three compilation errors, 7:06 each with the same description that we saw just a bit ago. 7:08 Looks like there are some additional calls to the viewModel.Init 7:12 method that we need to update. 7:15 Double-click the first error in the list to jump to that line of code, and 7:16 update the method to pass the _context private field. 7:20 And repeat twice more. 7:24 Now that our project is building, 7:37 let's test our changes to ensure that the database initializer works as expected. 7:39 I'll open the SQL Server Object Explorer window and 7:44 delete the ComicBookLibraryManager database. 7:47 Then, I'll run the app. 7:54 And after a bit of a delay while EF is busy recreating our database, 8:01 our list of comic books displays. 8:05 Clicking on a Detail button takes us to that comic book's detail page. 8:10 Click the Back button to return to the comic books list page, 8:15 then click the Add Comic Book button. 8:18 And here's the Add Comic Book page with the series. 8:22 Artist, and Role select list, populated with their respective available values. 8:29 By instantiating an instance of the context class, 8:38 in the ComicBooksController class constructor, we've aligned the beginning 8:40 of our context's lifetime with the beginning of the controller's lifetime. 8:44 But we still need to align the end of the context's lifetime 8:48 to the end of the controller's lifetime. 8:51 But how do we do that? 8:53 How can we dispose our context object instance when the controller is disposed? 8:55