实体框架 - 按一列分组并选择多列

Question

I have this method for my API where I'm trying to filter out a user ( userId ) and group by EventId . Then I want to select three columns from the table: name , eventId and UserId .

This is my code but it's not working:

[HttpGet("[action]/{userId}")]
public IActionResult EventsMatchFiltered(int userId)
{
    var events = _dbContext.Events.Where(event1 => event1.UserId != userId)
       .GroupBy(g => new {g.EventId })
       .SelectMany(g => g.Select(h => new 
       {
           Name = h.Name, 
           h.EventId, 
           h.UserId 
       }));
    return Ok(events);
}

Answer 1

Firstly, events would be an IQueryable rather than a materialized collection. Second, it would be an anonymous type rather than a proper DTO/ViewModel serializable class. Thirdly, you are grouping by an EventId, but then not "selecting" anything based on that grouping. EventId sounds like the Primary Key of the Event you are supposedly grouping.. Are you sure you don't actually mean to Group by User??

Normally for something like a Grouped result you would have a view model something like:

[Serializable]
public class UserEventSummaryViewModel
{
    public int UserId { get; set; }
    public ICollection<EventSummaryViewModel> Events { get; set; } = new List<EventSummaryViewModel>();
}

[Serializable]
public class EventSummaryViewModel
{
    public int EventId { get; set; }
    public string Name { get; set; }
}

then to query and fill:

var otherUserEvents = _dbContext.Events
   .Where(e => e.UserId != userId)
   .GroupBy(g => g.UserId )
   .Select(g => new UserEventSummaryViewModel
   { 
       UserId == g.Key,
       Events = g.Select(e => new EventSummaryViewModel
       {
           EventId = e.EventId,
           Name = e.Name
       }).ToList()
   }).ToList();

This would provide a structure of events grouped by each of the other Users. If you want UserId and UserName then add the UserName property to the UserEventSummary view model and in the GroupBy clause:

.GroupBy(g => new { g.UserId, g.Name } )

and populate these in the viewmodel with:

UserId == g.Key.UserId,
UserName = g.Key.Name,