id	summary	reporter	owner	description	type	status	component	version	severity	resolution	keywords	cc	stage	has_patch	needs_docs	needs_tests	needs_better_patch	easy	ui_ux
29340	"""cache.get_or_set()"" extra hits on database"	hematinik	nobody	"I've found out that using {{{cache.get_or_set()}}} causes extra SQL queries while using the alternative approach to get a key’s value or set a value if the key isn’t in the cache (using ""if"" statement) works fine without extra hits on database.
it seems the problem appears when you use the {{{cache.get_or_set()}}} inside a loop . this is the code to reproduce the problem:

THE MODEL:

{{{
#!python

class EventManager(models.Manager):

    def get_location(self,event):

        output = cache.get_or_set(
            'event_location_eID{}'.format(event.pk),
            event.location,
            None)

        return output

class Event(models.Model):

    objects = EventManager()
    location = models.ForeignKey(Location,on_delete = models.CASCADE)
    
}}} 

THE VIEW:
{{{
#!python
def home(request):

    events = cache.get_or_set('all_events',Event.objects.all(),None) # this line works fine without extra hits on db
    locations = []

    for event in events:
        x = Event.objects.get_location(event) # this line causes extra SQL queries while the cache keys already exist
        locations.append(x)

    return render(request,'index.html',{'locations':locations})
}}}

THE TEMPLATE:
{{{
#!python
{% for location in locations %}
    
    {{location.title}} <br>

{% endfor %}
}}}

now if I change the {{{get_location()}}} method on {{{EventManager}}} to this:

{{{
#!python
def get_location(self,event):

    output = cache.get('event_location_eID{}'.format(event.pk)) 
    if output is None:
        output = event.location
        cache.set('event_location_eID{}'.format(event.pk),output,None)

    return output
}}}

there will be no more unnecessarily SQL queries.

I'm using **django-debug-toolbar** to monitor my database requests and the information about queries is based on the statistics that **django-debug-toolbar** provides. I'll also attach some screenshots from  **django-debug-toolbar** to clarify the situation.

sql queries when using {{{get_or_set()}}}: [https://imgur.com/a/dydWEuL]
sql queries when using the alternative approach : [https://imgur.com/a/vYpfUZ3]
 
Regards.

"	Cleanup/optimization	closed	Core (Cache system)	2.0	Normal	invalid	cache		Unreviewed	0	0	0	0	0	0