Linq中的SQL在SQL中没有受支持的翻译

Question

I am trying to group by week in Linq. I created helper method to define the week I want to group by but when running the code I get the error message:

{"Method 'Int32 GetWeekNumber(System.DateTime)' has no supported translation to SQL."} System.SystemException {System.NotSupportedException}

My Linq query:

var data = from trans in _dbc.MyStuff
       where
           SqlMethods.Like(trans.SEGMENT2, "00601") && SqlMethods.Like(trans.SEGMENT4, "%")
           &&
           trans.CREATION_DATE >= new DateTime(2013, 01, 01) &&
           trans.CREATION_DATE <= new DateTime(2013, 12, 31)
       group trans by new
       {
           DateYear = (int?)trans.CREATION_DATE.Value.Year,
           DateMonth = (int?)trans.CREATION_DATE.Value.Month,
           DateWeek = (int?)WeekStuff.GetWeekNumber(trans.CREATION_DATE.Value)
       }
           into g
           orderby
               g.Key.DateYear,
               g.Key.DateMonth,
               g.Key.DateWeek
           select new
           {
               DateYear = g.Key.DateYear ?? 0,
               DateMonth = g.Key.DateMonth ?? 0,
               DateWeek = g.Key.DateWeek ?? 0,
               Value = g.Sum(p => p.BASE_TRANSACTION_VALUE) ?? 0,
               Count = g.Sum(p => p.PRIMARY_QUANTITY) ?? 0,
               DateCreated = g.Min(p => p.CREATION_DATE) ?? DateTime.Now,
               Department = g.Min(p => p.SEGMENT2)
           };

My Helper Method:

    public static int GetWeekNumber(DateTime date)
    {
        if (DateTimeFormatInfo.CurrentInfo != null)
        {
            Calendar cal = DateTimeFormatInfo.CurrentInfo.Calendar;
            return cal.GetWeekOfYear(date, CalendarWeekRule.FirstDay, DayOfWeek.Monday);
        }
        return 0;
    }

Basically what I want to do is group also by week, how would I group using helper method (if I in fact will need a helper method to accomplish this)?

Answer 1

LINQ tries to convert your code to sql and run it on the server. Your custom method can't be translated to SQL, that's normal. You'll probably have to populate the result of LINQ query first and then to group it additionally using LINQ to objects.

Answer 2

Step 1. Create a function in SQL that does the calculation needed (note this depends upon the @@DATEFIRST setting on your SQL server:

CREATE FUNCTION [dbo].[GetWeek](@dt datetime)
RETURNS int
BEGIN
    RETURN DATEPART( wk, @dt)
END

Step 2. In your data context class, add a function that does the same as your helper above, but marked as with the System.Data.Linq.Mapping.FunctionAttribute attribute:

[Function(Name="dbo.GetWeek", IsComposable=true)]
public int GetWeekNumber([Parameter(Name="@dt", DbType="datetime")]DateTime date)
{
  Calendar cal = DateTimeFormatInfo.CurrentInfo.Calendar;
  return cal.GetWeekOfYear(date, CalendarWeekRule.FirstDay, DayOfWeek.Monday);
}

The IsComposable=true indicates it can be used as part of other queries (ie it's a function, not a stored procedure).

Step 3. Now change your query to use dbc.GetWeekNumber instead of WeekStuff.GetWeekNumber .

Alternatively, you could define the above method as:

[Function(Name="dbo.GetWeek", IsComposable=true)]
public int GetWeekNumber([Parameter(Name="@dt", DbType="datetime")]DateTime date)
{
  return (int)ExecuteMethodCall(this, (MethodInfo)MethodInfo.GetCurrentMethod(), date).ReturnValue;
}

That way, even when called outside of a query, it will hit the database rather than calculating it in the .NET code. This is slower (hitting a database for what we could calculate without it), but has the advantage of keeping the concept of first-day-of-week consistent between code that hits the database, and code that does not.