Issue with an EF Core query

Question

I am trying to create a query for an entity that has a one-to-many relationship to another entity in EF Core .

Here are the two entities:

public class Computer {
  public int ComputerId { get; set; }
  public string Hostname { get; set; }
  public string IpAddress { get; set; }

  public ICollection<Event> Events { get; set; }
}

public class Event {
  public int EventId { get; set; }
  public string Description { get; set; }
  public DateTime EventTime { get; set; }

  public Computer Computer { get; set; }
}

I am trying to find out how I would go about querying all Computer records with the EventId of the newest Event .

I understand how to do simple queries on each, how would I go about writing a query that would return the data I need from both?

Answer 1

Newer versions of EF Core (5+) understand how to turn this into a ROW_NUMBER OVER, which is an efficient way to get what you're after:

context.Computers
  .Select(c => new {
    Computer = c, //or just take some properties from it
    RecentEvent = c.Events.OrderByDescending(e => e.EventTime).FirstOrDefault()
  });

It gets written to something like:

SELECT *
FROM
  Computers c
  INNER JOIN (
    SELECT *, ROW_NUMBER() OVER(PARTITION BY ComputerId ORDER BY EventTime DESC) rn
    FROM Events
  ) e ON c.ComputerId = e.ComputerId AND rn = 1

It's the "and rn=1" that does the magic

---

This gets the whole of each object, but you can select just the ID if you want (I assumed you wanted more data from the event).

Answer 2

Off the top of my head, you probably want something close to this. I might have my clauses ordered wrong, but I always need to monkey around with EF Core queries.

using var ctx = new MyDatabaseContext();

var newestEventsForEachComputer =
    ctx.Events
       .AsNoTracking()
       .Include(e => e.Computer)
       .OrderByDescending(e => e.EventTime)
       .GroupBy(e => e.Computer.Id)
       .Select(g => g.First())
       .ToList();

Note: I am assuming it is valid to have the .Include clause there given your description of the table linking.

Answer 3

You can try this, the result will be a list of Computers and they will have a list with only one Event with the latest id

dbContext.Computer.Include(x => x.Event.OrderByDescending(x => x.Id).Take(1)).Select(x => x).ToList();

Answer 4

First of all, you need to build a proper relationship between models. If you go with the code first approach. follow the below code snippets

 public class Computer
{
    [Key]
    public int ComputerId { get; set; }
    public string Hostname { get; set; }
    public string IpAddress { get; set; }

    [InverseProperty("Computer")]
    public ICollection<Event> Events { get; set; }
}


public class Event
{
    [Key]
    public int EventId { get; set; }
    public string Description { get; set; }
    public DateTime EventTime { get; set; }
    public Computer Computer { get; set; }
}

Now you have created a good relationship between the computer and the event model.

Now, the computer entity has a collection of its own events and those events has the same computer entity.

But you cannot get collection of computers just by querying a single eventId. Because your relationship is not like that. You can query like this

 [HttpGet]
    public async Task<IActionResult> GetByIdAsync(int id)
    {
        var result = await _context.Computers.Include(a => a.Events).FirstOrDefaultAsync(a => a.ComputerId == id);
        return new JsonResult(result);
    }

This will return a specific computer with related events.