在LINQ C＃中按日期分组

Question

I am trying to GROUPBY Date in a LINQ Query and display the output as shown below

startdates: [

startdate: “4/1/2014”,

users: [

{userId, …},
{userId, …}

],

startdate: “4/2/2014”, users: [

{userId, …}
],
 …

]

The code is shown below

db.Users
    .Where(x => (x.startDate >= startDate) && (x.startDate <= endDate))
    .GroupBy(x => new { x.startDate.Day, x.startDate.Month, x.startDate.Year })                    
    .ToList()
    .Select(y => new 
    {
    startdates = y.Select(k => 
    new {startdate = (k.startDate.Month.ToString() + "/" + k.startDate.Day.ToString() + "/" + k.startDate.Year.ToString()), 
    users = y.Select(z => 
    new {userId = z.userId,
    userName = z.userName})})});

Even though the Users are Grouped by StartDate, the output contains the startDate multiple times the same number of times as the number of Users.The output is shown below. I tried putting .Distinct() but it still repeats the startdate. Can someone please help?

[{"startdates":
[{"startdate":"04/01/2014",
"users":[
{"userId":1},"userName":"John"}
{"userId":2},"userName":"Mike"}],
[{"startdate":"04/01/2014",
"users":[
{"userId":1},"userName":"John"}
{"userId":2},"userName":"Mike"}],

[{"startdate":"04/02/2014",
"users":[
{"userId":3},"userName":"AL"}
{"userId":4},"userName":"Test"}],
[{"startdate":"04/02/2014",
"users":[
{"userId":3},"userName":"AL"}
{"userId":4},"userName":"Test"}]

Answer 1

The problem is your selection part, here:

.Select(y => new 
{
startdates = y.Select(k => 
new {startdate = (k.startDate.Month.ToString() + "/" + k.startDate.Day.ToString() + "/" + k.startDate.Year.ToString()), 
users = y.Select(z => 
new {userId = z.userId,
userName = z.userName})})});

You've got far too much nesting there. You're creating a startdate part for each element within the group.

It's unclear why you're using grouping by three separate parts at all, but I suspect this will do what you want:

db.Users
  .Where(x => (x.startDate >= startDate) && (x.startDate <= endDate))
  .GroupBy(x => x.startDate.Date) // Or just x.startDate 
  .AsEnumerable() // Do the rest of the query locally
  .Select(group => new 
  {
      startdate = group.Key.ToString("MM/dd/yyyy"),
      users = group.Select(z => new { z.userId, z.userName })
  });

If you need to wrap that in a startdates field, you can then use:

var result = new { startdates = query.ToArray() };