Django | How to make a reverse Relationship with two models - django

Question

i want to make a ticket system. I have the main model (TicketSystem) and a model with the messages from the users (TicketSystem_Messages).
In the model "TicketSystem_Messages" is the ForeignKey to the model "TicketSystem".
Here is my code:
class TicketSystem(models.Model):
subject = models.CharField(_('Subject'),max_length=30, blank=False, default="N/A")
message = models.TextField(_('Message'), null=False, blank=False)
created_date = models.DateTimeField(_('Date'), default=datetime.utcnow().replace(tzinfo=utc))
fertig_date = models.DateTimeField(_('Date'), default=datetime.utcnow().replace(tzinfo=utc))
class TicketSystem_Messages(models.Model):
user_name = models.ForeignKey(User)
status = models.ForeignKey(TicketSystem_Status)
ticketid = models.ForeignKey(TicketSystem)
message = models.TextField(_('Message'), null=False, blank=False)
created_date = models.DateTimeField(_('Sent'), default=datetime.utcnow().replace(tzinfo=utc))
At the moment i get the Tickets without the Messages:
sql_TicketSystem = TicketSystem.objects.filter(id=kwargs['pk'])
I want to make a LEFT JOIN like this
SELECT * FROM TicketSystem LEFT JOIN TicketSystem_Messages ON Ticketsystem.id = TicketSystem_Messages.ticketid
I heard something about "select_related" and "prefetch_related" and tried it but it does not work.

Answer 1

Rather than using raw SQL joins, in Django you can traverse model relationships in either direction, regardless of how they are represented in the database.
To find all the messages for a given TicketSystem instance:
my_ticket = TicketSystem.objects.get(id=0) # or something
my_ticket_messages = my_ticket.ticketsystem_messages_set.all() # or do other filters here
To find all the messages using a queryset:
TicketSystem_Messages.objects.filter(ticketId=my_ticket)
To find all tickets with more than one message:
from django.db import Count
TicketSystem.objects.annotate(message_count=Count('ticketsystem_messagess')).filter(message_count__gt=1)

Answer 2

If i want to list all tickets with the last status from the model "TicketSystem_Messages", it doesnt work. So with "TicketSystem.objects.get" it works without any problems.
Example:
sql_TicketSystem = TicketSystem.objects.all()
sql_TicketSystem_Messages = sql_TicketSystem.ticketsystem_messages_set.all()
UPDATE:
{% for data in sql_TicketSystem %}
{{ data.subject }}
{% for msg in data.ticketsystem_messages_set.all %}
{{msg.status}}
{% empty %}
Unknown
{% endfor %}
{% endfor %}
This works :-)