Entity Framework in ASP.NET MVC Application (三)

In the previous tutorial you implemented a set of web pages for basic CRUD operations for Student entities. In this tutorial you'll add sorting, filtering, and paging functionality to the Students Index page. You'll also create a page that does simple grouping.

The following illustration shows what the page will look like when you're done. The column headings are links that the user can click to sort by that column. Clicking a column heading repeatedly toggles between ascending and descending sort order.

Adding Column Sort Links to the Students Index Page

To add sorting to the Student Index page, you'll change the Index method of the Student controller and add code to the Student Index view.

Adding Sorting Functionality to the Index Method

In Controllers\StudentController.cs, replace the Index method with the following code:

This code receives a sortOrder parameter from the query string in the URL, which is provided by ASP.NET MVC as a parameter to the action method. The parameter will be a string that's either "Name" or "Date", optionally followed by a space and the string "desc" to specify descending order. 
The first time the Index page is requested, there's no query string. The students are displayed in ascending order by LastName, which is the default as established by the fall-through case in the switch statement. When the user clicks a column heading hyperlink, the appropriate sortOrder value is provided in the query string.
The two ViewBag variables are used so that the view can configure the column heading hyperlinks with the appropriate query string values:
These are ternary statements. The first one specifies that if the sortOrder parameter is null or empty, ViewBag.NameSortParm should be set to "Name desc"; otherwise, it should be set to an empty string.
There are four possibilities, depending on how the data is currently sorted:
If the current order is Last Name ascending, the Last Name link must specify Last Name descending, and the Enrollment Date link must specify Date ascending. If the current order is Last Name descending, the links must indicate Last Name ascending (that is, empty string) and Date ascending. If the current order is Date ascending, the links must indicate Last Name ascending and Date descending. If the current order is Date descending, the links must indicate Last Name ascending and Date ascending. 
The method uses LINQ to Entities to specify the column to sort by. The code creates an IQueryable variable before the switch statement, modifies it in the switch statement, and calls the ToList method after the switch statement. When you create and modify IQueryable variables, no query is sent to the database. The query is not executed until you convert the IQueryable object into a collection by calling a method such as ToList. Therefore, this code results in a single query that is not executed until the return View statement.
Adding Column Heading Hyperlinks to the Student Index View
In Views\Student\Index.cshtml, replace the <tr> and <th> elements for the heading row with the following code:
Adding a Search Box to the Students Index Page
To add filtering to the Student Index page, you'll add a text box and a submit button to the view and make corresponding changes in the Index method. The text box will let you enter a string to search for in the first name and last name fields.
Adding Filtering Functionality to the Index Method
In Controllers\StudentController.cs, replace the Index method with the following code:
You've added a searchString parameter to the Index method. You've also added a where clause to the LINQ statement that selects only students whose first name or last name contains the search string. The search string value is received from a text box that you'll add later to the Index view. The statement that adds the where clause is executed only if there's a value to search for:
Run the page, enter a search string, and click Search to verify that filtering is working.
Adding Paging to the Students Index Page
To add paging to the Student Index page, you'll start by installing the PagedList NuGet package. Then you'll make additional changes in the Index method and add paging links to the Index view. The following illustration shows the paging links.
Installing the PagedList NuGet Package
The NuGet PagedList package installs a PagedList collection type. When you put query results in a PagedList collection, several properties and methods are provided that facilitate paging.
In Visual Studio, make sure the project (not the solution) is selected. From the Tools menu, select Library Package Manager and then Add Library Package Reference.
In the Add Library Package Reference dialog box, click the Online tab on the left and then enter "pagedlist" in the search box. When you see the PagedList package, click Install.
Adding Paging Functionality to the Index Method
In Controllers\StudentController.cs, add a using statement for the PagedList namespace:
This code adds a page parameter, a current sort order parameter, and a current filter parameter to the method signature, as shown here:
The @model statement at the top of the page specifies that the view now gets a PagedList object instead of a List object.
The text box is initialized with the current search string so that the user can page through filter results without the search string disappearing:
Model.PageNumber is 1, and Model.PageCount is 0.)
Run the page.
Click the paging links in different sort orders to make sure paging works. Then enter a search string and try paging again to verify that paging also works correctly with sorting and filtering.
Creating an About Page That Shows Student Statistics
For the Contoso University website's About page, you'll display how many students have enrolled for each enrollment date. This requires grouping and simple calculations on the groups. To accomplish this, you'll do the following:
Create a view model class for the data that you need to pass to the view. Modify the About method in the Home controller. Modify the About view. Creating the View Model
Create a ViewModels folder. In that folder, create EnrollmentDateGroup.cs and replace the existing code with the following code:
You've now seen how to create a data model and implement basic CRUD, sorting, filtering, paging, and grouping functionality. In the next tutorial you'll begin looking at more advanced topics by expanding the data model.
Links to other Entity Framework resources can be found at the end of the last tutorial in this series.