Linq查询以选择不同的记录并计算不同的记录数

Question

I have table tblVisitor which stores visitors location details. It has below structure.

vRegion      NVARCHAR (100) NULL,
vTimeZone    NVARCHAR (100) NULL,
vLat         NVARCHAR (MAX) NULL, //Latitude
vLong        NVARCHAR (MAX) NULL, //Longitude

Now, this storage depends on each session and when the session expires the same location details might be stored again. Now I am trying to display this in a Map and thus am getting Latitude and Longitude details in a model class. My model class is as below:

public class VisitorMapViewModel
{
    public int count { get; set;}
    public GeoCoordinate coords { get; set; }
}

Am trying to fill model as below, but with no luck.

List<VisitorMapViewModel> model = new List<VisitorMapViewModel>();
var data = _db.tblVisitors.Distinct().ToList().Select(v => new VisitorMapViewModel()
            {
                coords = new System.Device.Location.GeoCoordinate(Convert.ToDouble(v.vLat), Convert.ToDouble(v.vLong)),
                count = _db.tblVisitors.GroupBy(m => new { m.vLat,m.vLong}).Select(m=>m).Distinct().Count()
            }).ToList();

There are 2 problems here.. Converting Latitude and Longitude values of type nvarchar to double and then getting distinct values of latitude and longitude and counting the number of occurances.. The above query keeps hitting one or other kind of exception when I interchange converting to ToList() before Distinct() and vice versa.. I feel am almost near, but not sure what am missing. Could anyone help on this please.

Answer 1

You should do one query that groups on the latitude and longitude and gets the counts. Then convert the results into your model. You can use AsEnumerable when you want to switch from SQL queries to in memory Linq queries without needing the overhead of creating a list.

var data = (from visitor in _db.tblVisitors
            group visitor by new { visitor.vLat, visitor.vLong } into grp
            select new 
            {
                grp.Key.vLat,
                grp.Key.vLong,
                Count = grp.Count()
            })
    .AsEnumerable()
    .Select(x => new new VisitorMapViewModel()
        {
            coords = new System.Device.Location.GeoCoordinate(
                double.Parse(x.vLat), 
                double.Parse(x.vLong)),
            count = x.Count
        })
    .ToList();

Alternatively you could do this all in method syntax

var data = _db.tblVisitors
    .GroupBy(v => new { v.vLat, v.vLong })
    .Select(grp => new 
        {
            grp.Key.vLat,
            grp.Key.vLong,
            Count = grp.Count()
        })
    .AsEnumerable()
    .Select(x => new new VisitorMapViewModel()
        {
            coords = new System.Device.Location.GeoCoordinate(
                double.Parse(x.vLat), 
                double.Parse(x.vLong)),
            count = x.Count
        })
    .ToList();