MVC LINQ 无法识别的方法

Question

我有一个从表中提取数据的 linq 查询，我想在给定一个日期的情况下将其转换为周（例如它是一年中的哪一周）。

还有 GetWeekofYear 函数：

 private int GetWeekOfYear(DateTime d)
 {
     var cal = System.Globalization.DateTimeFormatInfo.CurrentInfo.Calendar;
     return cal.GetWeekOfYear(new DateTime(d.Year, d.Month, 1), System.Globalization.CalendarWeekRule.FirstDay, System.DayOfWeek.Sunday);
 }

在当前状态下，当我尝试测试它（使用 Postman / Fiddler）时，我收到以下错误：

LINQ to Entities 无法识别方法 'Int32 GetWeekOfYear(System.DateTime)' 方法，并且该方法不能 翻译成商店表达式

Answer 1

出现该错误是因为 Linq2Sql 无法将GetWeekOfYear 方法转换为 SQL。

尝试以下方法：

• 选择原始数据而不是QuestionaireDetailsDTO

  select new QuestionaireDetailsDTO() {
      DepartureDate = transport.DepartureDate 
  };
  

• 向QuestionaireDetailsDTO 添加一个执行计算的getter：

  public string Week => GetWeekOfYear(DepartureDate);
  

这样转换发生在内存而不是数据库中。

如果 GetWeekOfYear 方法驻留在 DTO 的使用者无法访问的项目中，请在从 DB 中选择 DTO 后添加一个后处理步骤。

foreach (var result in query) {
     result.Week = GetWeekOfYear(result.DepartureDate);
}

Answer 2

你可以使用 .AsEnumerable() 来做到这一点

var query = from booking in context.BookingTables
                    join transport in context.TransportAllotments on booking.TransportAllotmentID equals transport.TransportAllotmentID
                    join passenger in context.Passengers on booking.BookingID equals passenger.BookingID
                    join result in context.QuestionaireResults on passenger.PassengerID equals result.PassengerID
                    join question in context.QuestionaireQuestions on result.QuestionaireQuestionID equals question.QuestionaireQuestionID
                    where transport.DepartureDate >= startDate && transport.DepartureDate <= endDate && booking.BookingID == id
                    .AsEnumerable()
                    select new QuestionaireDetailsDTO()
                    {
                        ID = booking.BookingID,
                        Date = transport.DepartureDate,
                        Question = question.QuestionText,
                        Week =  GetWeekOfYear(transport.DepartureDate) 

                    };

希望对您有所帮助。

Answer 3

正如我所说，这是因为 LinqToSQL 无法将您的方法转换为 SQL。 我认为@GeorgPatscheider 的答案不是最好的，因为您不应该更改数据对象，因为数据访问层的某些特定机制。 所以，@PhongDao 的回答更酷，但他的解决方案会从数据库中下载太多字段。你可以这样改变你的代码：

var query = from booking in context.BookingTables
        join transport in context.TransportAllotments on booking.TransportAllotmentID equals transport.TransportAllotmentID
        join passenger in context.Passengers on booking.BookingID equals passenger.BookingID
        join result in context.QuestionaireResults on passenger.PassengerID equals result.PassengerID
        join question in context.QuestionaireQuestions on result.QuestionaireQuestionID equals question.QuestionaireQuestionID
        where transport.DepartureDate >= startDate && transport.DepartureDate <= endDate && booking.BookingID == id
        // select only fields which we need
        select new
            {
                ID = booking.BookingID,
                Date = transport.DepartureDate,
                Question = question.QuestionText,
                DepartureDate = transport.DepartureDate
            }
        // retrieve data from DB
        .ToArray()
        // create items which you need
        .Select(x=>
        new QuestionaireDetailsDTO()
        {
            ID = x.ID,
            Date = x.Date,
            Question = x.Question,
            Week = GetWeekOfYear(x.DepartureDate)
        })
        // forming results
        .ToArray();