Linq - group by datetime for previous 12 months - include empty months

问题

I have a scenario whereby I need to retrieve a count of objects grouped by the month of a datetime field.

I found the following post which gets me part of the way there...

Linq: group by year and month, and manage empty months

...but I need to list the previous 12 months from today's date and the count of objects for each month, which is where I'm struggling.

I've seen a few other posts with similar issues/solutions but I chose the above one as it's also a requirement to produce a record for any months with a count of 0.

Thanks for any help I can get on this.

EDIT

OK, I got a little further thanks to Enigmativity (Thanks for taking the time!):

var news = from s in db.NewsItems
                   where s.SubmittedDate > first
                   select new 
                   {
                       Date = s.SubmittedDate,
                       Title = s.Title,
                   };

var grouping = from g in news.AsEnumerable()
                       select new NewsCountCollection
                       (
                           g.Date,
                           g.Title
                       );

var lookup = grouping.ToLookup(x => x.Month, x => x.Title);

var counts = from n in Enumerable.Range(-11, 12)
                    let Month = last.AddMonths(n)
                    select new
                    {
                        Month,
                        Count = lookup[Month].Count(),
                    };

var countList = from c in counts.AsEnumerable()
                        select new NewsCountMonthList
                        (
                            c.Month.ToString("MMMM"),
                            c.Count
                        );

...and the following

public class NewsCountCollection
{
    public DateTime Month { get; set; }
    public string Title { get; set; }

    public NewsCountCollection(DateTime date, string title)
    {
        this.Month = new DateTime(date.Year, date.Month, 1);
        this.Title = title;
    }

}

public class NewsCountMonthList
{
    public string Month { get; set; }
    public int Count { get; set; }

    public NewsCountMonthList(string month, int count)
    {
        this.Month = month;
        this.Count = count;
    }
}

...seems very inefficient though...I can't help thinking there must be a better way than this. Am I on the right track?

回答1:

This should do it for you:

var now = DateTime.Now;
var last = new DateTime(now.Year, now.Month, 1);
var first = last.AddMonths(-12);

var query =
    from s in somethings
    where s.DateTimeField >= first
    where s.DateTimeField < last
    select new
    {
        Month = new DateTime(s.DateTimeField.Year, s.DateTimeField.Month, 1),
        Something = s,
    };

var lookup = query.ToLookup(x => x.Month, x => x.Something);

var counts =
    from n in Enumerable.Range(-12, 12)
    let Month = last.AddMonths(n)
    select new
    {
        Month,
        Count = lookup[Month].Count(),
    };

You may need to fiddle with it a bit, but the structure should be sound.

来源：https://stackoverflow.com/questions/11188069/linq-group-by-datetime-for-previous-12-months-include-empty-months

标签

linq

datetime

group-by