从IQueryable查询中返回一条记录

本文关键字：一条 记录 返回 IQueryable 查询 | 更新日期: 2023-09-27 18:02:11

大家好，我有这个查询

  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，我怎么能返回第一个值作为IQueryable

从IQueryable查询中返回一条记录

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)(通常为null)，如果它是一个类对象…

所以要把它作为IQueryable，这对我来说似乎不像这样做:

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();

只需将您的查询包含在()中并附加Take(1)，这是FirstOrDefault的集合等效:

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