LINQ查询以按月将记录分组

Question

I am looking for some help on adapting the following LINQ query to return all dates within the next 6 months, even those where no records fall within the given month.

var maxDate = DateTime.Now.AddMonths(6);

var orders = (from ord in db.Items

where (ord.Expiry >= DateTime.Now && ord.Expiry <= maxDate)

group ord by new
    {
        ord.Expiry.Value.Year,
        ord.Expiry.Value.Month
    }
into g
select new ExpiriesOwnedModel
    {
        Month = g.Select(n => n.Expiry.Value.Month).First(),
        Quantity = g.Count()
    }).ToList();

I'd really appreciate any assistance or pointers on how best to implement this.

Answer 1

I'm not sure how well it'll interact with your database, but I'd do this as with a join:

var firstDaysOfMonths = Enumerable.Range(0, 7).Select(i =>
    new DateTime(DateTime.Today.Year, DateTime.Today.Month, 1).AddMonths(i));
var orders = firstDaysOfMonths.GroupJoin(
    db.Items,
    fd => fd,
    ord => new DateTime(ord.Expiry.Value.Year, ord.Expiry.Value.Month, 1),
    (fd, ords) => new { Month = fd.Month, Quantity = ords.Count() });

Note you may end up with an extra month where before you didn't (on the first day of the month?)

Answer 2

Stolen from Rawling's answer, if you prefer query syntax for group joins (I do):

var orders =
    from month in Enumerable.Range(0, 7)
        .Select(i => new DateTime(DateTime.Today.Year, DateTime.Today.Month, 1).AddMonths(i))
    join ord in db.Items
        on month equals new DateTime(ord.Expiry.Value.Year, ord.Expiry.Value.Month, 1)
        into ords
    select new { month.Month, Quantity = ords.Count() };

Answer 3

Alternative if it does not play nice with the database:

var rawGroups = db.Items.Where(item.Expiry >= DateTime.Now && ord.Expiry <= maxDate)
                        .GroupBy(item => new
                           {
                            item.Expiry.Value.Year,
                            item.Expiry.Value.Month
                           }, g => new ExpiriesOwnedModel()
                          {
                            Month = g.Key.Month,
                            Quantity = g.Count()
                          }).ToDictionary(model => model.Month);

var result = Enumerable.Range(DateTime.Now.Month,6)
                       .Select(i => i > 12 ? i - 12 , i)
                       .Select(i => rawGroups.Keys.Contains(i) ?
                                    rawGroups[i] :
                                    new ExpiriesOwnedModel() 
                                    { Month = i , Quantity = 0 });