Entity Framework navigation collections performance issue [duplicate] - entity-framework

Question

This question already has answers here:
EF: Create/Remove relation from Many-to-Many relations when `AutoDetectChangesEnabled` and `ProxyCreationEnabled` are disabled on DbContext
(3 answers)
Closed 3 years ago.
I have a many-to-many relation in Entity Framework, in the mapping, EF allow us to specify the keys and table name. In my example below, a user can have many device logs and a device log can belong to many users.
HasMany(e => e.DeviceLogs)
.WithMany(e => e.Users)
.Map(m => m.ToTable("UserDeviceLog")
.MapLeftKey("UserId")
.MapRightKey("LogId")
);
Now, when i try to remove a device log from a user:
using (var context = new MyDbContext())
{
var user = context.Users.Find(userId);
var deviceLog = context.DeviceLogs.Find(logId);
user.DeviceLogs.Remove(deviceLog);
context.SaveChanges();
}
I know that I'm going to delete a relation from table "UserDeviceLog", not the device log itself.
This above code snippet works, but the problem is Entity Framework will load all device logs and then remove one record from table UserDeviceLog.
My application is blocked while execute the above logic. From the screenshot you can see, there is about 23 seconds between the two SQL statements. For the specified user, it has about 20K records(the first SQL statement). I'm guessing it was taken by transfering these 20K records from SQL Server to my application which cause my application very slow.
While this is unnecessary, how can i avoid it? I don't want to write a plain SQL delete statement, I prefer a solution in EF way.
Thank you!

Answer 1

This will occur because by accessing user.DeviceLogs you will lazy-load that collection.
If UserDeviceLogs is not accessible in the DbContext as a DbSet (which it typically would not as a many-to-many joining table/entity)...
What should get you there would be:
using (var context = new MyDbContext())
{
var userDeviceLog = context.Users.Where(x => x.UserId == userId)
.SelectMany(x => x.DeviceLogs.Where(l => l.LogId ==logId))
.SingleOrDefault();
if(userDeviceLog == null)
return;
context.Entry(userDeviceLog).State = EntityState.Deleted;
context.SaveChanges();
}
If a DeviceLog refers to any reference that also must be deleted you will need to handle that deletion as well. But this should unlink a user from a device log without loading all links.