c# - 为什么 EF 无法按另一个集合的顺序对项目进行排序以及如何解决？

Question

我一直在尝试使用字符串列表对 LINQ 查询进行排序。我正在查询学生学期并包括学期类(class)。 代码抛出以下错误:

System.NotSupportedException: 'LINQ to Entities does not recognize the method 'Int32 IndexOf(System.String)' method, and this method cannot be translated into a store expression.'

我已经使用 DataTables 完成了这种排序，其中只有行，但 EF 和对象树中没有。

如何解决这个问题？

这是我的代码:

private static readonly List<string> SortingOrder =
   new List<string>()
   {
        "Fall",
        "Spring",
        "Summer"
   };

using (var context = new EUContext())
{
    var tmp = context.Terms.Include(x=>x.TermCourses)
        .OrderBy(x => x.AcademicYear)
        .ThenBy(x => SortingOrder.IndexOf(x.TermRegistered))
        .Where(x => x.StudentID == studentId && x.DepartmentID == departmentId);
    return tmp.ToList();
}

Answer 1

您正在尝试通过仅在您的代码/RAM 中可用的方法对数据库进行排序。您需要将排序控制权转移回本地计算机:

private static readonly List<string> SortingOrder =
   new List<string>()
   {
        { "Fall" },
        { "Spring" },
        { "Summer" }
   };

using (var context = new EUContext())
{
    // this part will query the database
    var tmp = context.Terms
                     .Include(x=>x.TermCourses)
                     .Where(x => x.StudentID == studentId && x.DepartmentID == departmentId)

                     // this will transfer controll to your local machine
                     .AsEnumerable() 

                     // sort on your local machine by the data you have in your RAM
                     .OrderBy(x => x.AcademicYear)
                     .ThenBy(x => SortingOrder.IndexOf(x.TermRegistered));

    return tmp.ToList();
}