LINQ to Entities group-by failure using .date

Question

I am trying to do a Linq group by on just the date part of a datetime field.

This linq statement works but it groups by the date and the time.

var myQuery = from p in dbContext.Trends
          group p by p.UpdateDateTime into g
          select new { k = g.Key, ud = g.Max(p => p.Amount) };

When I run this statement to group by just the date I get the following error

var myQuery = from p in dbContext.Trends
          group p by p.UpdateDateTime.Date into g   //Added .Date on this line
          select new { k = g.Key, ud = g.Max(p => p.Amount) };

The specified type member 'Date' is not supported in LINQ to Entities. Only initializers, entity members, and entity navigation properties are supported.

How can I do a group by for the date and not the date and time?

Answer 1

Use the EntityFunctions.TruncateTime method:

var myQuery = from p in dbContext.Trends
          group p by EntityFunctions.TruncateTime(p.UpdateDateTime) into g
          select new { k = g.Key, ud = g.Max(p => p.Amount) };

Answer 2

You can't use DateTime.Date in an Linq-to-Entities query. You either have group by the fields explicitly or create a Date field in the database. (I had the same problem - I used a Date field in the db, never looked back).

Answer 3

Possible solution here which follows the pattern:

var q = from i in ABD.Listitem
    let dt = p.EffectiveDate
    group i by new { y = dt.Year, m = dt.Month, d = dt.Day} into g
    select g;

So, for your query [untested]:

var myQuery = from p in dbContext.Trends
      let updateDate = p.UpdateDateTime
      group p by new { y = updateDate.Year, m = updateDate.Month, d = updateDate.Day} into g
      select new { k = g.Key, ud = g.Max(p => p.Amount) };