[翻译]ASP.NET MVC 3 开发的20个秘诀（九）[20 Recipes for Programming MVC 3]：对列表进行筛选

议题

当排序和分页不能帮助用户找到他们需要的结果，那就根据条件筛选内容的方式来帮助他们寻找。

解决方案

添加一个新的链接可以让用户选择预制的标准内容通过Linq Library对列表内容进行数据筛选。

讨论

添加筛选链接，需要修改Books视图、Index视图和BookController控制器。修改与前两个秘诀类似，添加HTML链接，内容必须是允许用户选择的内容。添加三个新链接：全部、新发布和即将发布。新发布的内容是最近两周发布的内容。即将发布的内容是预告未来发布的内容。

下面是新的Books和Index视图。三个链接中都会包含排序规则的参数（以便保持当前用户的排序选项），后两个链接会包含一个名为“filter”的新的变量参数。与分页链接类似，激活的筛选器只显示链接，未激活的显示文字链接，以标识用户当前选定的筛选器选项。为了在用户更改排序选项时也能保持当前筛选器的选项，也需要按照当前筛选器更新：

@model PagedList.IPagedList<MvcApplication4.Models.Book>

<h2>@MvcApplication4.Resources.Resource1.BookIndexTitle</h2>
<p>
    @Html.ActionLink("Create New", "Create")
</p>
<p>
    Show:
    @if (ViewBag.CurrentFilter != "")
    {
        @Html.ActionLink("All", "Index", new {
        sortOrder = ViewBag.CurrentSortOrder })
    }
    else
    {
        @:All
    }
    &nbsp; | &nbsp;
    @if (ViewBag.CurrentFilter != "NewReleases")
    {
        @Html.ActionLink("New Releases", "Index", new {
        filter = "NewReleases", sortOrder =
        ViewBag.CurrentSortOrder })
    }
    else
    {
        @:New Releases
    }
    &nbsp; | &nbsp;
    @if (ViewBag.CurrentFilter != "ComingSoon")
    {
        @Html.ActionLink("Coming Soon", "Index", new {
        filter = "ComingSoon", sortOrder =
        ViewBag.CurrentSortOrder })
    }
    else
    {
        @:Coming Soon
    }
</p>
@Html.Partial("_Paging")
<table>
    <tr>
        <th>
            @Html.ActionLink("Title", "Index", new {
            sortOrder = ViewBag.TitleSortParam,
            filter = ViewBag.CurrentFilter })
        </th>
        <th>
            @Html.ActionLink("Isbn", "Index", new {
            sortOrder = ViewBag.IsbnSortParam,
            filter = ViewBag.CurrentFilter })
        </th>
        <th>
            Summary
        </th>
        <th>
            @Html.ActionLink("Author", "Index", new {
            sortOrder = ViewBag.AuthorSortParam,
            filter = ViewBag.CurrentFilter })
        </th>
        <th>
            Thumbnail
        </th>
        <th>
            @Html.ActionLink("Price", "Index", new {
            sortOrder = ViewBag.PriceSortParam,
            filter = ViewBag.CurrentFilter })
        </th>
        <th>
            @Html.ActionLink("Published", "Index", new {
            sortOrder = ViewBag.PublishedSortParam,
            filter = ViewBag.CurrentFilter })
        </th>
        <th></th>
    </tr>
    @foreach (var item in Model)
    {
        <tr>
            <td>
                @Html.DisplayFor(modelItem => item.Title)
            </td>
            <td>
                @Html.DisplayFor(modelItem => item.Isbn)
            </td>
            <td>
                @Html.DisplayFor(modelItem => item.Summary)
            </td>
            <td>
                @Html.DisplayFor(modelItem => item.Author)
            </td>
            <td>
                @Html.DisplayFor(modelItem => item.Thumbnail)
            </td>
            <td>
                @Html.DisplayFor(modelItem => item.Price)
            </td>
            <td>
                @Html.DisplayFor(modelItem => item.Published)
            </td>
            <td>
                @Html.ActionLink("Edit",
                "Edit", new { id = item.ID }) |
                @Html.ActionLink("Details",
                "Details", new { id = item.ID }) |
                @Html.ActionLink("Delete",
                "Delete", new { id = item.ID })
            </td>
        </tr>
    }
</table>
@Html.Partial("_Paging")

在之前我们创建的分页链接的分部视图中，也需要做这样的修改。在下面的代码中，四个分页链接中也需要传送当前页码、排序规则和筛选器选项：

<p>
    @if (Model.HasPreviousPage)
    {
        @Html.ActionLink("<< First", "Index", new {
        page = 1,
        sortOrder = ViewBag.CurrentSortOrder,
        filter = ViewBag.CurrentFilter })

        @Html.Raw("&nbsp;");
                           
        @Html.ActionLink("< Prev", "Index", new {
        page = Model.PageNumber - 1,
        sortOrder = ViewBag.CurrentSortOrder,
        filter = ViewBag.CurrentFilter })
    }
    else
    {
        @:<< First
        @Html.Raw("&nbsp;");
        @:< Prev
    }
    &nbsp;&nbsp;
    @if (Model.HasNextPage)
    {
        @Html.ActionLink("Next >", "Index", new {
        page = Model.PageNumber + 1,
        sortOrder = ViewBag.CurrentSortOrder,
        filter = ViewBag.CurrentFilter })
        
        @Html.Raw("&nbsp;");
        
        @Html.ActionLink("Last >>", "Index", new {
        page = Model.PageCount,
        sortOrder = ViewBag.CurrentSortOrder,
        filter = ViewBag.CurrentFilter })
    }
    else
    {
        @:Next >
        @Html.Raw("&nbsp;")
        @:Last >>
    }
</p>

接下来，再对BooksController中的Index()方法进行修改。接受一个新的筛选器变量，根据用户选择的筛选器，将会减少书籍列表的数量。有以下两种实现方式：

1. 为Linq添加Where的字符串选择条件；
2. 使用标准Linq的强类型选择条件。

由于通常筛选器通常不会包含太多条目，所以在这个秘诀中我们将使用第二种方法。使用第二种方法的时候，因为条目是预制的且是强类型的并不是动态的，所以不需要额外的输入检查或SQL注入防治。

using System;
using System.Collections.Generic;
using System.Data;
using System.Data.Entity;
using System.Linq;
using System.Linq.Dynamic;
using System.Web;
using System.Web.Mvc;
using MvcApplication4.Models;
using MvcApplication4.Utils;
using PagedList;

namespace MvcApplication4.Controllers
{
    public class BooksController : Controller
    {
        private BookDBContext db = new BookDBContext();
        //
        // GET: /Books/

        public ViewResult Index(string sortOrder,
        string filter, int page = 1)
        {
            #region ViewBag Resources
            ...
            #endregion
            #region ViewBag Sort Params
            ...
            #endregion
            var books = from b in db.Books select b;
            #region Filter Switch
            switch (filter)
            {
                case "NewReleases":
                    var startDate =      DateTime.Today.AddDays(-14);
                    books = books.Where(b => b.Published
                    <= DateTime.Today.Date
                    && b.Published >= startDate
                    );
                    break;
                case "ComingSoon":
                    books = books.Where(b => b.Published >
                    DateTime.Today.Date);
                    break;
                default:
                    // No filter needed
                    break;
            }
            ViewBag.CurrentFilter =
            String.IsNullOrEmpty(filter) ? "" : filter;
            #endregion
            books = books.OrderBy(sortOrder);
            int maxRecords = 1;
            int currentPage = page - 1;
            return View(books.ToPagedList(currentPage,
            maxRecords));
        }
        ...
    }

}

在上面的例子中，用户可以筛选最新的内容并搜索返回今天或过去14天发布的书籍，又或者用户也可以选择即将发布并搜索返回任何今天出版的书籍。否则，没有选择任何筛选器就会返回所有书籍。

参考

原书地址 书籍源代码