F () Expressing strange behavior with Django
I have this template tag that ultimately returns a list of active ads (checks if Campaign with an active field True
and then pulls ads from the Campaign with a request)
@register.assignment_tag
def get_current_campaigns(amount):
# Get all the campaigns that are active
current_campaigns = Campaign.objects.filter(active=True)
current_campaigns_count = current_campaigns.count()
# To avoid the list index being out of range and throwing an IndexError
# We reduce the amount to match the amount of rows in the model if the
# amount of rows is less than the amount being requested.
if amount > current_campaigns_count:
amount = current_campaigns_count
# Select active campaigns randomly
random_camps = []
for i in range(amount):
random_camps.append(random.choice(current_campaigns))
# prepare all the ads to return
output = []
for campaign in random_camps:
# get all the ads that a campaign has
ads = campaign.advertisement_set.all()
# now select one randomly
ad = random.choice(ads)
# hand it to output
output.append(ad)
# mark that this campaign has been seen
campaign.impressions = F('impressions') + 1
campaign.save()
# checks and sets if the campaign is still active
campaign.check_active()
return output
And here's the model that comes with it:
class Campaign(models.Model):
''' Represents an Advertisement Campaign '''
title = models.CharField(max_length=50, blank=True, verbose_name='Campaign Title')
impressions = models.IntegerField()
impression_limit = models.IntegerField()
created = models.DateTimeField(auto_now_add=True)
active = models.BooleanField(default=False)
def check_active(self):
''' Checks if the Campaign is currently active '''
if self.impressions >= self.impression_limit:
self.active = False
self.save()
Weird bit: every time I visit a page that has an ad enabled, and then check it in the admin, object impressions are incremented by 2 (it should be 1) and are marked as False, even if this one is if self.impressions >= self.impression_limit
not true, it doesn't matter how This changes the active field to False
independently.
Is there any clue why this strange behavior is happening? If necessary, I can provide additional information.
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random.choice
does not guarantee that you will receive non-repeating items.
import random random_camps = random.sample(current_campaigns, amount)
- the way here.
Update If you are worried about speed this question , we recommend picking random lines quickly in postgres.
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