limiting __in lookup in django - django

Question

i have are question about "__in" lookup in Django ORM.
So here is the example of code:
tags = [Tag.objects.get(name="sometag")]
servers = Server.objects.filter(tags__in=tags)[offset_from:offset_to]
server_infos = []
for server in servers:
server_infos.append(query.last())
So here is the problem: we making about 60-70 sql requests for each server. But i want to do something like this:
tags = [Tag.objects.get(name="sometag")]
servers = Server.objects.filter(tags__in=tags)[offset_from:offset_to]
server_infos = ServerInfo.objects.filter(contains__in=servers)
assert servers.count() == server_infos.count()
Can i do this without raw sql request? All i need to understand is how to limit "__in" expression in Django to get only last value as in example above. Is it possible?
Update, my models:
class Tag(models.Model):
name = models.CharField(max_length=255, blank=True)
added_at = models.DateTimeField(auto_now_add=True, null=True)
def __str__(self):
return self.name
class Server(models.Model):
ip = models.CharField(max_length=255, blank=True)
port = models.IntegerField(blank=True)
name = models.CharField(max_length=255, blank=True)
tags = models.ManyToManyField(Tag)
added_at = models.DateTimeField(auto_now_add=True, null=True)
def __str__(self):
return self.name
def get_server_online(self):
query = ServerInfo.objects.filter(contains=self)
if query.exists():
return query.last().online
return 0
class ServerInfo(models.Model):
contains = models.ForeignKey(Server, \
on_delete=models.CASCADE, null=True, blank=True)
map = models.CharField(max_length=255, blank=True)
game = models.CharField(max_length=255, blank=True)
online = models.IntegerField(null=True)
max_players = models.IntegerField(null=True)
outdated = models.BooleanField(default=False)
tags = models.ManyToManyField(Tag)
ping = models.IntegerField(null=True)
def __str__(self):
return f"Current map {self.map} and current online {self.online}/{self.max_players}"

Answer 1

I think the problem is that tags is ManyToMany and a server may be selected twice by two different tags. Also that a server may have >1 serverinfos, because it's a ForeignKey relation not a OneToOne.
Possibilities:
Make sure a server is returned only once in the queryset:
servers = Server.objects.filter(tags__in=tags).distinct()[offset_from:offset_to]
(or distinct('pk') ?)
Make sure only one ServerInfo instance is returned per server:
server_infos = ServerInfo.objects.filter(contains__in=servers).distinct('contains')
Or use prefetch_related in the Servers query, and then avoid subsequent queries by always referring to the serverinfo objects through the related name (default "serverinfo_set")
I absolutely hate "magic" default related names, and would always code one explicitly: contains = models.ForeignKey(Server, ..., related_name='server_infos', ...)
servers = Server.objects.filter(tags__in=tags).distinct(
).prefetch_related('serverinfo')[offset_from:offset_to]
for server in servers:
server_info = server.serverinfo_set.first()
# or
for info in server.serverinfo_set:
NB don't start applying filters to serverinfo_set if you don't want to hit the DB N times. Filter by iterating through what is presumably a short list, and which is already in memory in the queryset anyway.

Answer 2

The following should retrieve the data using two queries, one to get all the servers and the second one to get all the server infos using prefetch_related(). When filtering on a ManyToManyField you have to use distinct() to avoid duplicate results.
servers = Server.objects.filter(
tags__name="sometag").distinct().prefetch_related("server_info_set")
for server in servers:
print(server.name)
for info in server.server_info_set.all():
print(info)
If you only want to retrieve one certain info per server you would have to provide a custom queryset to prefetch_related() using Prefetch() objects.
from django.db.models import Prefetch
servers = Server.objects.filter(
tags__name="sometag").distinct().prefetch_related(Prefetch('server_info_set',
queryset=ServerInfo.objects.filter(outdated=False),
to_attr="current_infos"
)
)
for server in servers:
print(server.name)
for info in server.current_infos.all():
print(info)