Entity Framework - Linq query with order by and group by

Question

I have Measurement Objects with the relevant Properties CreationTime  DateTime  and Reference  String  and some other values   I d like to write an efficient linq query to a DbContext that    groups my Measurement objects by a given Reference orders the groups by either the CreationTime Property of the first Measurement from the group or the average of the CreationTime properties limits the query to the most recent numOfEntries    In a later step i calculate averaged values over these returned groups  but i guess that s not relevant for my problem    The DbContext itself has a DbSet lt Measurement gt  called Measurementsholding all Measurement objects   I came up with the following query  that results in a List of groups that is ordered correctly but is missing some groups in between   var groupByReference    from m in context Measurements                           orderby m CreationTime                           group m by new   m Reference   into g                           select g  Take numOfEntries  ToList      How do I select the  most recent  groups of Measurements correctly   I m using Entity Framework 4 4 with a MySQL Database  MySql Connector Net 6 6 4   This example is simplified  i can go into more detail and or give an example if necessary   Thanks

User · Answer

Your requirements are all over the place, but this is the solution to my understanding of them:

To group by Reference property:

var refGroupQuery = (from m in context.Measurements
            group m by m.Reference into refGroup
            select refGroup);

Now you say you want to limit results by "most recent numOfEntries" - I take this to mean you want to limit the returned Measurements... in that case:

var limitedQuery = from g in refGroupQuery
                   select new
                   {
                       Reference = g.Key,
                       RecentMeasurements = g.OrderByDescending( p => p.CreationTime ).Take( numOfEntries )
                   }

To order groups by first Measurement creation time (note you should order the measurements; if you want the earliest CreationTime value, substitue "g.SomeProperty" with "g.CreationTime"):

var refGroupsOrderedByFirstCreationTimeQuery = limitedQuery.OrderBy( lq => lq.RecentMeasurements.OrderBy( g => g.SomeProperty ).First().CreationTime );

To order groups by average CreationTime, use the Ticks property of the DateTime struct:

var refGroupsOrderedByAvgCreationTimeQuery = limitedQuery.OrderBy( lq => lq.RecentMeasurements.Average( g => g.CreationTime.Ticks ) );

User · Answer

You can try to cast the result of GroupBy and Take into an Enumerable first then process the rest  building on the solution provided by NinjaNye   var groupByReference    from m in context Measurements                                GroupBy m   gt  m Reference                                 Take numOfEntries  AsEnumerable                                   Select g   gt  new  Creation   g FirstOrDefault   CreationTime                                                Avg   g Average m   gt  m CreationTime Ticks                                                   Items   g                                   OrderBy x   gt  x Creation                                 ThenBy x   gt  x Avg                                 ToList   select m     Your sql query would look similar  depending on your input  this  SELECT TOP  3   t1   Reference  AS  Key  FROM       SELECT  t0   Reference      FROM  Measurements  AS  t0      GROUP BY  t0   Reference        AS  t1  GO  -- Region Parameters DECLARE  x1 NVarChar 1000     Ref1  -- EndRegion SELECT  t0   CreationTime    t0   Id    t0   Reference  FROM  Measurements  AS  t0  WHERE  x1    t0   Reference  GO  -- Region Parameters DECLARE  x1 NVarChar 1000     Ref2  -- EndRegion SELECT  t0   CreationTime    t0   Id    t0   Reference  FROM  Measurements  AS  t0  WHERE  x1    t0   Reference

User · Answer

Try moving the order by after group by   var groupByReference    from m in context Measurements                         group m by new   m Reference   into g                         order by g Avg i   gt  i CreationTime                          select g  Take numOfEntries  ToList

User · Answer

It s method syntax  which I find easier to read  but this might do it  Updated post comment  Use  FirstOrDefault   instead of  First    With regard to the dates average  you may have to drop that ordering for the moment as I am unable to get to an IDE at the moment  var groupByReference   context Measurements                                GroupBy m   gt  m Reference                                 Select g   gt  new  Creation   g FirstOrDefault   CreationTime                                                   Avg   g Average m   gt  m CreationTime Ticks                                                   Items   g                                   OrderBy x   gt  x Creation                                 ThenBy x   gt  x Avg                                 Take numOfEntries                                 ToList

[c#] Entity Framework - Linq query with order by and group by

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