Get value in 1-to-many relationship (EF Core) - asp.net-core

Question

I'm seeking an equivalent to this SQL query in EF Core:
SELECT Id, Name, (Select AdminRoleId From EventAdmins Where EventId = Events.Id And AdminId = [value from cookie]) As EventRoleId From Events
This is what I have so far:
public IList<Event> Events { get; set; }
public IList<EventAdmin> EventAdmins { get; set; }
public async Task<IActionResult> OnGetAsync() {
var adminId = Guid.Parse(Request.Cookies["Adm_AdminId"]);
Events = await _context.Events.SelectMany(e => e.EventAdmins.Where(x => x.EventId == e.Id && x.AdminId == adminId).Select(x => x.AdminRoleId)).AsNoTracking().ToListAsync();
return Page();
}
I'm not sure what's wrong, but I get an error saying "Error CS0452: The type 'Guid' must be a reference type in order to use it as parameter 'TEntity' in the generic type or method".
Event model:
public class Event {
public Guid Id { get; set; }
public string Name { get; set; }
[ForeignKey("Id")]
public IList<EventAdmin> EventAdmins { get; set; }
}
EventAdmin model:
public class EventAdmin {
public Guid Id { get; set; }
public Guid EventId { get; set; }
public Guid AdminId { get; set; }
public Guid AdminRoleId { get; set; }
[ForeignKey("EventId")]
public Event Events { get; set; }
}

Answer 1

Reason
This error occurs because you're trying to ask EF Core not to track a list of GUID. However, the list of Guid is a list of value types.
As you know, EF Core can only track a series of reference type, so the method signature of AsNoTracking<TEntity>() is :
public static IQueryable<TEntity> AsNoTracking<TEntity> (this IQueryable<TEntity> source)
where TEntity : class;
Note the constraints of where TEntity : class.
In other words, you can never invoke AsNoTracking<Guid>():
Events = await _context.Events
.SelectMany(e => e.EventAdmins.Where(x => x.EventId == e.Id).Select(x => x.AdminRoleId))
.AsNoTracking() // Actually, it will invoke `AsNoTracking<Guid>()`
.ToListAsync();
How to Fix
Your SQL doesn't seem valid. I guess you want to return a {Id, Name, EventRoleId}.
If you would like to do that with SelectMany, you could simply query as below:
var Events = await this._context.Events
.SelectMany(
e => e.EventAdmins.Where(x => x.EventId == e.Id).Select(x => x.AdminRoleId),
(p,g) => new {
Id = p.Id,
Name = p.Name,
EventRoleId = g
}
)
// .AsNoTracking()
.ToListAsync();
There's no need to call .AsNoTracking() at all. Because no tracking is performed if the result set does not contain any entity types.
As a side note, you shouldn't decorate the Event.EventAdmins with a [ForeignKey("Id")] attribute :
public class Event {
public Guid Id { get; set; }
public string Name { get; set; }
[ForeignKey("Id")]
public IList EventAdmins { get; set; }
}
Because the Event is the principal entity and the EventAdmin is the dependent entity. Only the EventAdmin has a foreign key that references the Event.

Answer 2

Your LINQ query doesn't seem to include adminId so I can't see how it could work. Try something like this:
var eventAdmin = _context.EventAdmin.SingleOrDefault( e => e.Id == adminId);
var events = eventAdmin.Events;
Try using a tool like Linqpad to dissect your queries one step at a time.