Question

I’m using the .NET and the Entity Framework.
I want to group a table by a date without the time part.

I’ve got the class:

public sealed class QueryItem {
    public int Year { get; set; }
    public int Month { get; set; }
    public int Day { get; set; }
}

And the method (I’m using the BLToolkit, if you want to know what is DbManager):

protected override IQueryable<QueryItem> InitiateQuery() {
    return
        from query in DbManager.GetTable<SomeTableModel>()
        group query by new { Year = query.CreationDate.Year, Month = query.CreationDate.Month, Day = query.CreationDate.Day }
        into list1
        select new QueryItem {Year = list1.Key.Year, Month = list1.Key.Month, Day = list1.Key.Day};
}

In this method linq translated into this SQL query:

SELECT
    [query].[CreationDate]
FROM
    [SomeDatabase.SomeTable] [query]
GROUP BY
    DatePart(Year, [query].[CreationDate]),
    DatePart(Month, [query].[CreationDate]),
    DatePart(Day, [query].[CreationDate]),
    [query].[CreationDate]

And this is wrong query because it doesn’t group only by a date.

But if I use this code (it’s not usable, I need to return a class object):

var result =
    from query in DbManager.GetTable<SomeTableModel>()
    group query by new { Year = query.CreationDate.Year, Month = query.CreationDate.Month, Day = query.CreationDate.Day }
    into list1
    select list1;

It will be translated into this:

SELECT
    [t1].[c1] as [c11],
    [t1].[c2] as [c21],
    [t1].[c3] as [c31]
FROM
    (
        SELECT
            DatePart(Year, [selectParam].[CreationDate]) as [c1],
            DatePart(Month, [selectParam].[CreationDate]) as [c2],
            DatePart(Day, [selectParam].[CreationDate]) as [c3]
        FROM
            [SomeDatabase.SomeTable] [selectParam]
    ) [t1]
GROUP BY
    [t1].[c1],
    [t1].[c2],
    [t1].[c3]

And this is the right query.

I suppose that Entity Framework trying to optimise my query or something like this and this is why I’ve got the wrong query.

Am I right?
Am I doing it in the right way?
What should I do (maybe use direct SQL query or something else)?

Answer 1

It is much simpler to use EntityFunctions* here:

var result =
    from query in DbManager.GetTable<SomeTableModel>()
    group query by EntityFunctions.TruncateTime(query.CreationDate)
    into list1
    select list1;

^{*DbFunctions as of Entity Framework 6.}