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排序、筛选、分页以及分组
Sorting, filtering, paging, and grouping¶
By Tom Dykstra
The Contoso University sample web application demonstrates how to create ASP.NET Core 1.0 MVC web applications using Entity Framework Core 1.0 and Visual Studio 2015. For information about the tutorial series, see the first tutorial in the series.
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.
Sections:
- Add Column Sort Links to the Students Index Page
- Add a Search Box to the Students Index page
- Add paging functionality to the Students Index page
- Add paging functionality to the Index method
- Add paging links to the Student Index view
- Create an About page that shows Student statistics
- Summary
Add Column Sort Links to the Students Index Page 在Students Index页面中增加列排序链接
To add sorting to the Student Index page, you’ll change the Index method of the Students controller and add code to the Student Index view.
Add sorting Functionality to the Index method
In StudentsController.cs, replace the Index method with the following code:
public async Task<IActionResult> Index(string sortOrder){ ViewData["NameSortParm"] = String.IsNullOrEmpty(sortOrder) ? "name_desc" : ""; ViewData["DateSortParm"] = sortOrder == "Date" ? "date_desc" : "Date"; var students = from s in _context.Students select s; switch (sortOrder) { case "name_desc": students = students.OrderByDescending(s => s.LastName); break; case "Date": students = students.OrderBy(s => s.EnrollmentDate); break; case "date_desc": students = students.OrderByDescending(s => s.EnrollmentDate); break; default: students = students.OrderBy(s => s.LastName); break; } return View(await students.AsNoTracking().ToListAsync());}
This code receives a sortOrder parameter from the query string in the URL. The query string value is provided by ASP.NET Core MVC as a parameter to the action method. The parameter will be a string that’s either “Name” or “Date”, optionally followed by an underscore and the string “desc” to specify descending order. The default sort order is ascending.
此代码从 URL 中的查询字符串接收 sortOrder 参数。查询字符串的值由ASP.NET Core MVC 作为该方法的参数而提供。该参数将是一个字符串，可以是Name或者Date，可选情况是，后面跟着一条下划线和"desc"来指定降序排列。默认排序顺序升序。
The first time the Index page is requested, there’s no query string. The students are displayed in ascending order by last name, 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.
当索引页第一次被请求时，并没有查询字符串。学生默认情况下按姓氏降序排列，这在 switch 语句中缺省按升序排列。当用户单击列标题的超链接时，查询字符串就提供了适当 sortOrder 值。
The two ViewData elements (NameSortParm and DateSortParm) are used by the view to configure the column heading hyperlinks with the appropriate query string values.
public async Task<IActionResult> Index(string sortOrder){ ViewData["NameSortParm"] = String.IsNullOrEmpty(sortOrder) ? "name_desc" : ""; ViewData["DateSortParm"] = sortOrder == "Date" ? "date_desc" : "Date"; var students = from s in _context.Students select s; switch (sortOrder) { case "name_desc": students = students.OrderByDescending(s => s.LastName); break; case "Date": students = students.OrderBy(s => s.EnrollmentDate); break; case "date_desc": students = students.OrderByDescending(s => s.EnrollmentDate); break; default: students = students.OrderBy(s => s.LastName); break; } return View(await students.AsNoTracking().ToListAsync());}
These are ternary statements. The first one specifies that if the sortOrder parameter is null or empty, NameSortParm should be set to “name_desc”; otherwise, it should be set to an empty string. These two statements enable the view to set the column heading hyperlinks as follows:
| Current sort order | Last Name Hyperlink | Date Hyperlink |
|---|---|---|
| Last Name ascending | descending | ascending |
| Last Name descending | ascending | ascending |
| Date ascending | ascending | descending |
| Date descending | ascending | 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 ToListAsync 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 ToListAsync. Therefore, this code results in a single query that is not executed until the return View statement.
Add column heading hyperlinks to the Student Index view¶
Replace the code in Views/Students/Index.cshtml, with the following code to rearrange the column order and add column heading hyperlinks. The new column headings are highlighted.
@model IEnumerable<ContosoUniversity.Models.Student>@{ ViewData["Title"] = "Index";}<h2>Index</h2><p> <a asp-action="Create">Create New</a></p><table class="table"> <thead> <tr> <th> <a asp-action="Index" asp-route-sortOrder="@ViewBag.NameSortParm">@Html.DisplayNameFor(model => model.LastName)</a> </th> <th> @Html.DisplayNameFor(model => model.FirstMidName) </th> <th> <a asp-action="Index" asp-route-sortOrder="@ViewBag.DateSortParm">@Html.DisplayNameFor(model => model.EnrollmentDate)</a> </th> <th></th> </tr> </thead> <tbody> @foreach (var item in Model) { <tr> <td> @Html.DisplayFor(modelItem => item.LastName) </td> <td> @Html.DisplayFor(modelItem => item.FirstMidName) </td> <td> @Html.DisplayFor(modelItem => item.EnrollmentDate) </td> <td> <a asp-action="Edit" asp-route-id="@item.ID">Edit</a> | <a asp-action="Details" asp-route-id="@item.ID">Details</a> | <a asp-action="Delete" asp-route-id="@item.ID">Delete</a> </td> </tr> } </tbody></table>This code uses the information in ViewData properties to set up hyperlinks with the appropriate query string values.
Run the page and click the Last Name and Enrollment Date column headings to verify that sorting works.
Add a Search Box to the Students Index page¶
To add filtering to the Students 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.
Add filtering functionality to the Index method¶
In StudentsController.cs, replace the Index method with the following code (the changes are highlighted).
public async Task<IActionResult> Index(string sortOrder, string searchString){ ViewData["NameSortParm"] = String.IsNullOrEmpty(sortOrder) ? "name_desc" : ""; ViewData["DateSortParm"] = sortOrder == "Date" ? "date_desc" : "Date"; ViewData["CurrentFilter"] = searchString; var students = from s in _context.Students select s; if (!String.IsNullOrEmpty(searchString)) { students = students.Where(s => s.LastName.Contains(searchString) || s.FirstMidName.Contains(searchString)); } switch (sortOrder) { case "name_desc": students = students.OrderByDescending(s => s.LastName); break; case "Date": students = students.OrderBy(s => s.EnrollmentDate); break; case "date_desc": students = students.OrderByDescending(s => s.EnrollmentDate); break; default: students = students.OrderBy(s => s.LastName); break; } return View(await students.AsNoTracking().ToListAsync());}You’ve added a searchString parameter to the Index method. The search string value is received from a text box that you’ll add to the Index view. You’ve also added to the LINQ statement a where clause that selects only students whose first name or last name contains the search string. The statement that adds the where clause is executed only if there’s a value to search for.
Note
Here you are calling the Where method on an IQueryable object, and the filter will be processed on the server. In some scenarios you might be calling the Where method as an extension method on an in-memory collection. (For example, suppose you change the reference to _context.Students so that instead of an EF DbSet it references a repository method that returns an IEnumerable collection.) The result would normally be the same but in some cases may be different.
For example, the .NET Framework implementation of the Contains method performs a case-sensitive comparison by default, but in SQL Server this is determined by the collation setting of the SQL Server instance. That setting defaults to case-insensitive. You could call the ToUpper method to make the test explicitly case-insensitive: Where(s => s.LastName.ToUpper().Contains(searchString.ToUpper()). That would ensure that results stay the same if you change the code later to use a repository which returns an IEnumerable collection instead of an IQueryable object. (When you call the Contains method on an IEnumerable collection, you get the .NET Framework implementation; when you call it on an IQueryable object, you get the database provider implementation.) However, there is a performance penalty for this solution. The ToUpper code would put a function in the WHERE clause of the TSQL SELECT statement. That would prevent the optimizer from using an index. Given that SQL is mostly installed as case-insensitive, it’s best to avoid the ToUpper code until you migrate to a case-sensitive data store.
Add a Search Box to the Student Index View¶
In Views/Student/Index.cshtml, add the highlighted code immediately before the opening table tag in order to create a caption, a text box, and a Search button.
<p> <a asp-action="Create">Create New</a> </p> <form asp-action="Index" method="get"> <div class="form-actions no-color"> <p> Find by name: <input type="text" name="SearchString" value="http://www.mamicode.com/@ViewData["currentFilter"]" /> <input type="submit" value="http://www.mamicode.com/Search" class="btn btn-default" /> | <a asp-action="Index">Back to List</a> </p> </div> </form> <table class="table">This code uses the <form> tag helper to add the search text box and button. By default, the <form> tag helper submits form data with a POST, which means that parameters are passed in the HTTP message body and not in the URL as query strings. When you specify HTTP GET, the form data is passed in the URL as query strings, which enables users to bookmark the URL. The W3C guidelines recommend that you should use GET when the action does not result in an update.
Run the page, enter a search string, and click Search to verify that filtering is working.
Notice that the URL contains the search string.
http://localhost:5813/Students?SearchString=an
If you bookmark this page, you’ll get the filtered list when you use the bookmark. Adding method="get" to the form tag is what caused the query string to be generated.
At this stage, if you click a column heading sort link you’ll lose the filter value that you entered in the Search box. You’ll fix that in the next section.
Add paging functionality to the Students Index page¶
To add paging to the Students Index page, you’ll create a PaginatedList class that uses Skip and Take statements to filter data on the server instead of always retrieving all rows of the table. Then you’ll make additional changes in the Index method and add paging buttons to the Index view. The following illustration shows the paging buttons.
In the project folder create PaginatedList.cs, and then replace the template code with the following code.
using System;using System.Collections.Generic;using System.Linq;using System.Threading.Tasks;using Microsoft.EntityFrameworkCore;public class PaginatedList<T> : List<T>{ public int PageIndex { get; private set; } public int TotalPages { get; private set; } public PaginatedList(List<T> items, int count, int pageIndex, int pageSize) { PageIndex = pageIndex; TotalPages = (int)Math.Ceiling(count / (double)pageSize); this.AddRange(items); } public bool HasPreviousPage { get { return (PageIndex > 1); } } public bool HasNextPage { get { return (PageIndex < TotalPages); } } public static async Task<PaginatedList<T>> CreateAsync(IQueryable<T> source, int pageIndex, int pageSize) { var count = await source.CountAsync(); var items = await source.Skip((pageIndex - 1) * pageSize).Take(pageSize).ToListAsync(); return new PaginatedList<T>(items, count, pageIndex, pageSize); }}The CreateAsync method in this code takes page size and page number and applies the appropriate Skip and Take statements to the IQueryable. When ToListAsync is called on the IQueryable, it will return a List containing only the requested page. The properties HasPreviousPage and `HasNextPage can be used to enable or disable Previous and Next paging buttons.
A CreateAsync method is used instead of a constructor to create the PaginatedList<T> object because constructors can’t run asynchronous code.
Add paging functionality to the Index method¶
In StudentsController.cs, replace the Index method with the following code.
public async Task<IActionResult> Index( string sortOrder, string currentFilter, string searchString, int? page){ ViewData["CurrentSort"] = sortOrder; ViewData["NameSortParm"] = String.IsNullOrEmpty(sortOrder) ? "name_desc" : ""; ViewData["DateSortParm"] = sortOrder == "Date" ? "date_desc" : "Date"; if (searchString != null) { page = 1; } else { searchString = currentFilter; } ViewData["CurrentFilter"] = searchString; var students = from s in _context.Students select s; if (!String.IsNullOrEmpty(searchString)) { students = students.Where(s => s.LastName.Contains(searchString) || s.FirstMidName.Contains(searchString)); } switch (sortOrder) { case "name_desc": students = students.OrderByDescending(s => s.LastName); break; case "Date": students = students.OrderBy(s => s.EnrollmentDate); break; case "date_desc": students = students.OrderByDescending(s => s.EnrollmentDate); break; default: students = students.OrderBy(s => s.LastName); break; } int pageSize = 3; return View(await PaginatedList<Student>.CreateAsync(students.AsNoTracking(), page ?? 1, pageSize));