BLToolKit, Linq Query, SQL Not what I expected
Pergunta
I am using BLToolKit in a project of mine and I was trying to get this to work. What I don't like is that I am trying to average a bunch of temps down to the minute, but the select statement that is being generated groups by the minute but then selects the original time. I think I am doing the linq expression correctly (but then again, i am not getting the results i expect). (this is C#, if you care) Anyone know what is going wrong?
var test = (from r in db.SensorReadingRaws
where r.TimeLogged < DateTime.Now.AddMinutes(-2)
group r by new
{
Sensor = r.SensorNumber,
//group time down to the minute
Time = r.TimeLogged.AddSeconds(-1 * r.TimeLogged.Second).AddMilliseconds(-1 * r.TimeLogged.Millisecond)
} into grouped
select new SensorReading
{
SensorNumber = grouped.Key.Sensor,
TimeLogged = grouped.Key.Time,
Reading = (int)grouped.Average(x => x.Reading)
}).ToList();
textBox1.Text = db.LastQuery;
and the resulting query is this
SELECT
[r].[SensorNumber],
[r].[TimeLogged],
Avg([r].[Reading]) as [c1]
FROM
[SensorReadingRaw] [r]
WHERE
[r].[TimeLogged] < @p1
GROUP BY
[r].[SensorNumber],
DateAdd(Millisecond, Convert(Float, -DatePart(Millisecond, [r].[TimeLogged])), DateAdd(Second, Convert(Float, -DatePart(Second, [r].[TimeLogged])), [r].[TimeLogged])),
[r].[TimeLogged]
Solução
I discovered that
BLToolkit.Data.Linq.Sql.AsSql<T>(T obj)
can be used as a workaround for this case.
When applying this function to the required grouped key properties in select statement you get rid of grouping/selecting an original field.
It may look something like:
_queryStore.Leads().
GroupBy(x => new {
x.LeadDate.Hour,
x.LeadDate.Minute
}).
Select(x => new {
Hour = Sql.AsSql(x.Key.Hour),
Minute = Sql.AsSql(x.Key.Minute),
Count = x.Count()
});
and in your particular case:
var test = (from r in db.SensorReadingRaws
where r.TimeLogged < DateTime.Now.AddMinutes(-2)
group r by new
{
Sensor = r.SensorNumber,
//group time down to the minute
Time = r.TimeLogged.AddSeconds(-1 * r.TimeLogged.Second).AddMilliseconds(-1 * r.TimeLogged.Millisecond)
} into grouped
select new SensorReading
{
SensorNumber = grouped.Key.Sensor,
TimeLogged = Sql.AsSql(grouped.Key.Time),
Reading = (int)grouped.Average(x => x.Reading)
}).ToList();
Outras dicas
I got same issue yesterday. Today I found a workaround. The idea is to write 2 linq queries. First transforming the data and the second grouping the result:
var bandAndDate =
(from r in repo.Entities
select new {Band = r.Score / 33, r.StartTime.Date});
var examsByBandAndDay =
(from r in bandAndDate
group r by new {r.Band, r.Date } into g
select new { g.Key.Date, g.Key.Band, Count = g.Count() }).ToList();
Both this queries run one SQL that do the job:
SELECT
[t1].[c1] as [c11],
[t1].[c2] as [c21],
Count(*) as [c3]
FROM
(
SELECT
[r].[Score] / 33 as [c2],
Cast(Floor(Cast([r].[StartTime] as Float)) as DateTime) as [c1]
FROM
[Results] [r]
) [t1]
GROUP BY
[t1].[c2],
[t1].[c1]