Entity Core and SaveChanges works only once - entity-framework-core

Question

I have problem with Entity Core 3.1.2.
I have code like this:
SQL.Database.EnsureCreated();
var ThisCollector = SQL.CollectorServers
.Where(esa => esa.ServerName == ServerCollectorName)
.FirstOrDefault();
while (foo)
{
await SQL.Entry(ThisCollector)
.ReloadAsync();
DateTime dtTimeOut = DateTime.UtcNow.AddMinutes(-1);
//check status of current Worker
var CurrentLB = SQL.CollectorServers
.Where(esa => esa.isWorker == true
&& esa.LastSeenLB < dtTimeOut)
.FirstOrDefault();
if (CurrentLB!=null) //Current Worker is dead!
{
CurrentLB.isWorker = false;
ThisCollector.isWorker = true;
SQL.SaveChanges(); //This works allways
}
var Collectors = SQL.CollectorServers
.Where(Esa => Esa.isWorker == true);
if (Collectors.Count() == 0)
{
ThisCollector.isWorker = true;
SQL.SaveChanges();
}
if (Collectors.Count() >= 2)
{
foreach(CollectorServer cs in Collectors)
{
cs.isWorker = false;
//why this is requied?
SQL.Entry(cs).State = Microsoft.EntityFrameworkCore.EntityState.Modified;
}
ThisCollector.isWorker = true;
//<--This works only once, without manually setting State to
// modified!!! Why? Values has been changed from external program.
//(Management studio in this case)
SQL.SaveChanges();
}
await Task.Delay(10000, stoppingToken);
}
Problem is that last SaveChanges works only first time it has been called without I set Entry state to Modified. After that it does not make SQL query (I can see that in SQL Profiler).
In this case this can be fixed by this way, but I'm trying to undestand why this happends. My software saves lot data to SQL, and I need to know can I trust to this code without opening all queries and adding this modified state.
I didin't have this kind of problems in full version (6) of Entity, this is something quite new for me.

Answer 1

The described behavior would make sense if the entities returned by CollectorServers are not tracked.
ThisCollector is attached on every loop by the call to SQL.Entry(ThisCollector) :
await SQL.Entry(ThisCollector) //Attaching
.ReloadAsync(); //Reloading
Any changes made to it would be tracked and saved by the first call to DbContext.SaveChanges().
On the other hand, the entities returned by the Collectors query :
var Collectors = SQL.CollectorServers
.Where(Esa => Esa.isWorker == true);
Would remain untracked, until they get reattached by the call to SQL.Entry(cs) :
foreach(CollectorServer cs in Collectors)
{
cs.isWorker = false;
SQL.Entry(cs).State = Microsoft.EntityFrameworkCore.EntityState.Modified;
}
That's equivalent to calling DbContext.Update to attach and set the state to Modified. Update is easier to read though :
foreach(CollectorServer cs in Collectors)
{
cs.isWorker = false;
SQL.Update(cs);
}