//In SelectGenerator
public override SqlFragment Visit(DbGroupByExpression expression)
{
    // first process the input
    DbGroupExpressionBinding e = expression.Input;
    SelectStatement innerSelect = VisitInputExpressionEnsureSelect(e.Expression, e.VariableName, e.VariableType);
    SelectStatement select = WrapIfNotCompatible(innerSelect, expression.ExpressionKind);

    CollectionType ct = (CollectionType)expression.ResultType.EdmType;
    RowType rt = (RowType)ct.TypeUsage.EdmType;

    int propIndex = 0;
    foreach (DbExpression key in expression.Keys)
    {
        var tmp = key.Accept(this) as ColumnFragment;
        select.AddGroupBy(tmp);
        propIndex++;
        
/* Sorry for my English.
 * we must add  to the 'select' expression each field, which is involved in 'groupby'.
 * Each field can be selected and we do not know exactly which of them (without analysis).
 * Simply select all. The main thing is how you are building the alias for the fields. I don't know.
 * 
 * 
 * 
 */

        tmp = tmp.Clone(); 
        tmp.ColumnAlias = String.Format("K{0}", propIndex); 

        select.Columns.Add(tmp); 
    }

    for (int agg = 0; agg < expression.Aggregates.Count; agg++)
    {
        DbAggregate a = expression.Aggregates[agg];
        DbFunctionAggregate fa = a as DbFunctionAggregate;
        if (fa == null) throw new NotSupportedException();

        string alias = rt.Properties[propIndex++].Name;
        ColumnFragment functionCol = new ColumnFragment(null, null);
        functionCol.Literal = HandleFunction(fa, a.Arguments[0].Accept(this));
        functionCol.ColumnAlias = alias;
        select.Columns.Add(functionCol);
    }
    return select;
}

//more correctly be
foreach (DbExpression key in expression.Keys)
{
    var tmp = key.Accept(this);
    select.AddGroupBy(tmp);
    propIndex++;

    var keyTmp = tmp as ColumnFragment;

    if(keyTmp != null)
    {
        keyTmp = keyTmp.Clone(); 
        keyTmp.ColumnAlias = String.Format("K{0}", propIndex); 

        select.Columns.Add(keyTmp);
    }        

     
}


//In class ColumnFragment
public ColumnFragment Clone()
{
    ColumnFragment cf = new ColumnFragment(TableName, ColumnName);
    cf.ColumnAlias = ColumnAlias;
    cf.TableAlias = TableAlias; //(in your v 6.4.0.0 this line is missing)
    cf.Literal = Literal;
    return cf;
}