NHibernate查询每组最新的N.

时间:2013-02-06 07:15:35

标签: nhibernate fluent-nhibernate linq-to-nhibernate

我通过FluentNHibernate映射了以下域

class Point
{
    public int Id { get; set; }
    public string Label { get; set; }
}

class PointData
{
    public int Id { get; set; }        
    public DateTime Hour { get; set; } 
    public decimal Value { get; set; }
    public Point Location { get; set; }
    public DateTime Available { get; set; } // Data arrival (data tracking)
}

PointData表会频繁更新一小时插入新值(不支持更新),但并非所有点都以相同的频率写入PointData。

我需要在startTime和endTime之间以及timeOfSnapshot(可用)之前的所有小时内获得每点Point的最新PointData。

到目前为止,我使用group by使用Linq到Nhibernate的查询不起作用:

Query.Where(x => x.Available <= available && x.Hour >= startHour && x.Hour < endHour)
                    .OrderByDescending(x => x.Available)
                    .GroupBy(x => new {x.Location, x.Hour})
                    .Select(g => g.First())
                    .ToList();

到目前为止,所有搜索都只提出了SQL查询,这不是我想要的。

帮助是适当的

2 个答案:

答案 0 :(得分:1)

使用原始行按结果加入组并返回行:

rows.Join(
   rows.GroupBy(x => new {x.Location, x.Hour}), 
   x => new {x.Location, x.Hour, x.Available}, 
   g => new {g.Key.Location, g.Key.Hour, Available = g.Max(p=>p.Available)},
   (x1,g1) => x1
)

打开一个分析器,你会看到这个sql:

SELECT [t0].*
FROM [rows] AS [t0]
INNER JOIN (
    SELECT MAX([t1].[Available]) AS [value], [t1].[Location], [t1].[Hour]
    FROM [rows] AS [t1]
    GROUP BY [t1].[Location], [t1].[Hour]) 
AS [t2] ON (
    [t0].[Location] = [t2].[Location]) 
    AND ([t0].[Hour] = [t2].[Hour]) 
    AND ([t0].[Available] = [t2].[value]
)

答案 1 :(得分:0)

在分组后,您无法提取比分组键或聚合中使用的列更多的列。如果我正确理解你的问题,通常的模式是使用一个子查询来选择感兴趣的行的标识符,然后使用外部查询来获取这些行的所有列。尝试这样的事情:

var latestPointId =
    Query.Where(x => x.Available <= available && x.Hour >= startHour && x.Hour < endHour)
        .OrderByDescending(x => x.Available)
        .GroupBy(x => new {x.Location, x.Hour})
        .Select(g => g.Max(pd => pd.Id));

session.Query<PointData>
    .Where(pd => latestPointId.Contains(pd.Id)
    .ToList();