[翻译][MVC 5 + EF 6] 3：排序、过滤、分页

原文：Sorting, Filtering, and Paging with the Entity Framework in an ASP.NET MVC Application

1.添加排序：

1.1.修改ControllersStudentController.cs的Index：

public ActionResult Index(string sortOrder)
{
   ViewBag.NameSortParm = String.IsNullOrEmpty(sortOrder) ? "name_desc" : "";
   ViewBag.DateSortParm = sortOrder == "Date" ? "date_desc" : "Date";
   var students = from s in db.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(students.ToList());
}

1.2.修改ViewsStudentIndex.cshtml：

<p>
    @Html.ActionLink("Create New", "Create")
</p>
<table>
    <tr>
        <th>
            @Html.ActionLink("Last Name", "Index", new { sortOrder = ViewBag.NameSortParm })
        </th>
        <th>First Name
        </th>
        <th>
            @Html.ActionLink("Enrollment Date", "Index", new { sortOrder = ViewBag.DateSortParm })
        </th>
        <th></th>
    </tr>

　　效果图：

2.添加搜索框：

2.1.修改ControllersStudentController.cs的Index：

        public ViewResult Index(string sortOrder, string searchString)
        {
            ViewBag.NameSortParm = String.IsNullOrEmpty(sortOrder) ? "name_desc" : "";
            ViewBag.DateSortParm = sortOrder == "Date" ? "date_desc" : "Date";
            var students = from s in db.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(students.ToList());
        }

　　在大多数情况下，我们调用同一个方法时，在EF实体集和内存中集合的扩展方法返回的结果是一样的;但是在一些情况下返回结果会不一样。

　　例如当传递一个空的字符串时.NET Framework的Contains方法返回所有行，但是SQL Server Compact 4.0的EF 提供者（provider）的Contains返回0行。因此上面的代码中把Where语句放在if语句中，以保证对所有的SQL Server版本返回相同的数据。同样，.NET Framework的Contains方法默认是区分大小写的，因此，使用ToUpper方法将字符串显示转换为大写以确保以后代码改为仓储时（返回类型由IQueryable类型变成IEnumerable类型）返回结果不会发生变化（当调用Contains方法返回IEnumerable集合时，调用的是.NET Framework实现；返回IQueryable对象时，调用的是数据库提供者的实现）。

　　当我们返回IQueryable对象时，不同的数据库提供者对空值的处理也不同。例如，在有些脚本中where条件中的table.Column != 0可能不会返回空值列。

2.2.为ViewsStudentIndex.cshtml添加搜索框：

<p>
    @Html.ActionLink("Create New", "Create")
</p>
@using (Html.BeginForm())
{
    <p>
        Find by name: @Html.TextBox("SearchString")
        <input type="submit" value="Search" />
    </p>
}

　　效果图：

3.添加分页：

3.1.安装PagedList.MVC：

Install-Package PagedList.Mvc

3.2.修改ControllersStudentController.cs：

using PagedList;

public ViewResult Index(string sortOrder, string currentFilter, string searchString, int? page)
{
   ViewBag.CurrentSort = sortOrder;
   ViewBag.NameSortParm = String.IsNullOrEmpty(sortOrder) ? "name_desc" : "";
   ViewBag.DateSortParm = sortOrder == "Date" ? "date_desc" : "Date";
　　
   if (searchString != null)
   {
      page = 1;
   }
   else
   {
      searchString = currentFilter;
   }

   ViewBag.CurrentFilter = searchString;

var students = from s in db.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:  // Name ascending 
         students = students.OrderBy(s => s.LastName);
         break;
   }

   int pageSize = 3;
   int pageNumber = (page ?? 1);
   return View(students.ToPagedList(pageNumber, pageSize));
}

　　当页面第一次加载或者用户没有点击分页或排序链接时，所有的参数都是空值。

　　ViewBag.CurrentSort用于点击分页时保存排序信息。

　　ViewBag.CurrentFilter用于保存搜索信息，并且当页面刷新时给搜索框赋值。当搜索条件改变时，page值重置为1。

3.3.修改ViewsStudentIndex.cshtml：

@model PagedList.IPagedList<ContosoUniversity.Models.Student>
@using PagedList.Mvc;
<link href="~/Content/PagedList.css" rel="stylesheet" type="text/css" />

@{
    ViewBag.Title = "Students";
}

<h2>Students</h2>

<p>
    @Html.ActionLink("Create New", "Create")
</p>
@using (Html.BeginForm("Index", "Student", FormMethod.Get))
{
    <p>
        Find by name: @Html.TextBox("SearchString", ViewBag.CurrentFilter as string)
        <input type="submit" value="Search" />
    </p>
}
<table class="table">
    <tr>
        <th>
            @Html.ActionLink("Last Name", "Index", new { sortOrder = ViewBag.NameSortParm, currentFilter=ViewBag.CurrentFilter })
        </th>
        <th>
            First Name
        </th>
        <th>
            @Html.ActionLink("Enrollment Date", "Index", new { sortOrder = ViewBag.DateSortParm, currentFilter=ViewBag.CurrentFilter })
        </th>
        <th></th>
    </tr>


@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>
            @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>
<br />
Page @(Model.PageCount < Model.PageNumber ? 0 : Model.PageNumber) of @Model.PageCount

@Html.PagedListPager(Model, page => Url.Action("Index", 
    new { page, sortOrder = ViewBag.CurrentSort, currentFilter = ViewBag.CurrentFilter }))

　　BeginForm提交form数据的默认方式是post，post方式是通过HTTP消息体而不是通过URL查询字符串传递参数。我们通过FormMethod.Get把它指定为HTTP GET,这样用户就可以将URL收藏为书签。当动作不会导致更新时W3C HTTP GET使用指南推荐我们使用GET。

　　效果图：

4.添加About页面展示Student统计信息：

4.1.创建视图模型：

　　新建ViewModels文件夹EnrollmentDateGroup.cs：

    public class EnrollmentDateGroup
    {
        [DataType(DataType.Date)]
        public DateTime? EnrollmentDate { get; set; }

        public int StudentCount { get; set; }
    }

4.2.修改HomeController.cs：

private SchoolContext db = new SchoolContext();
//...
public ActionResult About()
{
    IQueryable<EnrollmentDateGroup> data = from student in db.Students
               group student by student.EnrollmentDate into dateGroup
               select new EnrollmentDateGroup()
               {
                   EnrollmentDate = dateGroup.Key,
                   StudentCount = dateGroup.Count()
               };
    return View(data.ToList());
}
//...
protected override void Dispose(bool disposing)
{
    db.Dispose();
    base.Dispose(disposing);
}

4.3.修改ViewsHomeAbout.cshtml：

@model IEnumerable<ContosoUniversity.ViewModels.EnrollmentDateGroup>
           
@{
    ViewBag.Title = "Student Body Statistics";
}

<h2>Student Body Statistics</h2>

<table>
    <tr>
        <th>
            Enrollment Date
        </th>
        <th>
            Students
        </th>
    </tr>

@foreach (var item in Model) {
    <tr>
        <td>
            @Html.DisplayFor(modelItem => item.EnrollmentDate)
        </td>
        <td>
            @item.StudentCount
        </td>
    </tr>
}
</table>

　　效果图：