Linq，为每个服务选择最新条目

Question

I have the following table:

public class ServiceStatusHistory
{
    [Key]
    [Required]
    public int Id { get; set; }

    // Forenkey to service
    [Required]
    public virtual Service Service { get; set; }

    [Required]
    public ServiceStatus Status { get; set; }

    [Required]
    public string Messages { get; set; }

    [Required]
    //Time Logged in the db
    public DateTime time { get; set; }

    [Required]
    //Time the service last called the update method on the client
    public DateTime LastUpdateTime { get; set; }
}

Now I need to do a linq query that will.

Give me a list of ServiceStatusHistory Objects, for each service ID that there is in the db and only the latest(biggest time) one(for that service) where enabled = true.

The reason that this is complicated is that, there will be multiple entries in the table for each service. So a service with Id 4 could have 100 entries in here. and there will be multiple services.

here is my attempt:

 using (var db = new EFDbContext())
            {
                var result = (from x in db.ServiceStatusHistory
                              where x.Service.Enabled == true
                              select x).FirstOrDefault();

                list =  result.toString();
            }

But I realised my query will only return one object not a list of them?

Answer 1

You could group by the service id and then select the last of the groupings.

using (var db = new EFDbContext())
{
    var results = d.ServiceStatusHistory.Where(h => h.Service.Enabled)
                                        .OrderByDescending(h => h.time )
                                        .GroupBy(h => h.Service.Id)
                                        .Select(grp => grp.FirstOrDefault()); 
}

The OrderByDescending will give you the proper ordering for the FirstOrDefault . GroupBy preserves the ordering of the elements passed.

Results will be an IEnumerable<ServiceStatusHistory> where you will have no repeated Service based on the Id property of the class. Change the OrderByDescending if your "last" has a specific meaning.

Edit: To eagerly include an object, you can use the Include extension method:

d.ServiceStatusHistory.Include(h => h.Service)

Answer 2

You can try change to this:

  (from x in db.ServiceStatusHistory
                                  where x.Service.Enabled == true
                                  select x).Last();

Answer 3

Try this

(from x in db.ServiceStatusHistory
                                  where x.Service.Enabled == true
                                  select x).OrderBydescending(x => x.yourfeild).Top(1);

Answer 4

db.ServiceStatusHistory
    .Where(x => x.Status.Enabled)
    .GroupBy(x => x.Service)
    .Select(x => x.OrderByDescending(y => y.time).First());