Trouble with Select_for_update. Postgresql

Hi there . code is as follows:

`

models.Classroom table have just one record.

# THREAD 1

school_classrooms = models.Classroom.objects.select_for_update().all()

with transaction.atomic():

list(school_classrooms)

time.sleep(10)

# THREAD 2

classroom = models.Classroom.objects.first()

classroom.name = 'somename'

classroom.save()

`
thread 1 starts. then thread 2 starts to update the only Classroom record but because the record locked by table 1  , is will wait 10 seconds . then thread 1 releases the lock so thread 2 starts to update and exits with no error. surprisingly thread 2 didn't do anything at all and the Classroom record does not updated . How is is possible and how to solve that ? please help. thanks

database: Postgres in docker / latest version
django: django 3
disclaimer: thread 1 performed with a postman request to my api and thread 2 performed by django shell.

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