从 IQueryable 查询返回一条记录

Question

大家好，我有这个问题

  public IQueryable<HeaderMRC> ShowHeader(int MRCId)
        {
            return from m in _ctx.MaterialRequestContractorDetails
                   where m.MaterialRequestContractorId == MRCId
                   join materialRequestContractor in _ctx.MaterialRequestContractors on m.MaterialRequestContractorId equals materialRequestContractor.Id

                   join mat in _ctx.MaterialDescriptions on m.MaterialDescriptionId equals mat.Id
                   join l in _ctx.Lines on m.LineId equals l.Id
                   join s in _ctx.Sheets on l.Id equals s.LineId
                   select new HeaderMRC()
                   {
                       Code = materialRequestContractor.Code,
                       UnitArea = l.Unit,
                       LineType = l.Type,
                       RequestDate = materialRequestContractor.RequestDate
                   };

        }

这个查询产生多个记录，但我只需要第一个，如何将第一个值返回为 IQueryable

Answer 1

只需添加.First() 或.FirstOrDefault()

return (from m in _ctx.MaterialRequestContractorDetails
               where m.MaterialRequestContractorId == MRCId
               join materialRequestContractor in _ctx.MaterialRequestContractors on m.MaterialRequestContractorId equals materialRequestContractor.Id

               join mat in _ctx.MaterialDescriptions on m.MaterialDescriptionId equals mat.Id
               join l in _ctx.Lines on m.LineId equals l.Id
               join s in _ctx.Sheets on l.Id equals s.LineId
               select new HeaderMRC()
               {
                   Code = materialRequestContractor.Code,
                   UnitArea = l.Unit,
                   LineType = l.Type,
                   RequestDate = materialRequestContractor.RequestDate
               }).FirstOrDefault();

或

return (from m in _ctx.MaterialRequestContractorDetails
               where m.MaterialRequestContractorId == MRCId
               join materialRequestContractor in _ctx.MaterialRequestContractors on m.MaterialRequestContractorId equals materialRequestContractor.Id

               join mat in _ctx.MaterialDescriptions on m.MaterialDescriptionId equals mat.Id
               join l in _ctx.Lines on m.LineId equals l.Id
               join s in _ctx.Sheets on l.Id equals s.LineId
               select new HeaderMRC()
               {
                   Code = materialRequestContractor.Code,
                   UnitArea = l.Unit,
                   LineType = l.Type,
                   RequestDate = materialRequestContractor.RequestDate
               }).First();

FirstOrDefault 将返回第一个，或者如果没有 Default(T)，如果它是类对象，则通常为 null...

因此，将其作为 IQueryable 在我看来并不喜欢这样做：

return new List<HeaderMRC> { (from m in _ctx.MaterialRequestContractorDetails
               where m.MaterialRequestContractorId == MRCId
               join materialRequestContractor in _ctx.MaterialRequestContractors on m.MaterialRequestContractorId equals materialRequestContractor.Id

               join mat in _ctx.MaterialDescriptions on m.MaterialDescriptionId equals mat.Id
               join l in _ctx.Lines on m.LineId equals l.Id
               join s in _ctx.Sheets on l.Id equals s.LineId
               select new HeaderMRC()
               {
                   Code = materialRequestContractor.Code,
                   UnitArea = l.Unit,
                   LineType = l.Type,
                   RequestDate = materialRequestContractor.RequestDate
               }).FirstOrDefault() }.AsQueryable();

Answer 2

只需用() 括起您的查询并附加Take(1)，这是FirstOrDefault 的集合等效项：

return (from m in ...
        ...
        ...).Take(1);