Incrementing a variable of a class based on initiation of another class python/django

My goal is to display the total number of entries in a specific contest. Initially I tried doing this within the template section by incrementing a counter for each contest as I looped through entries. My thought is that when an entry submission is created, the total_submissions for the corresponding contest should be incremented, however I'm not sure how to do it.

Model:

class Contest(models.Model):
contest_name = models.CharField(max_length=200)
contest_description = models.CharField(max_length=5000)
contest_start_date = models.DateTimeField('start date')
contest_end_date = models.DateTimeField('end date')
submission_count = models.IntegerField(default=0)

def __str__(self):
return self.contest_name


class Submission(models.Model):
contest = models.ForeignKey(Contest, on_delete=models.CASCADE)
user = models.ForeignKey(User, on_delete=models.SET_NULL, null=True, blank=True)
date_uploaded = models.DateTimeField(auto_now_add=True)
votes = models.IntegerField(default=0)

def __init__(self):
#increment the submission_count variable for the corresponding contest


Other attempt by using templates:

<h1>Current Contests</h1>
<div class="row">
% if current_contest_list %
% for contest in current_contest_list %
<div class="col-sm-4"><h1><a href="% url 'contests:detail' contest.id%"> contest.contest_name </a></h1>
<img src="media/contest.display_image" class="img-responsive" alt="image">
contest.contest_name

% for submission in submission_list %

% if submission.contest == contest.contest_name %
<P>Hi!</P>
% endif %
<p> submission.contest contest.contest_name </p>
% if submission.contest == contest.contest_name %
<P>Hi!</P>
forloop.counter
% set total_submissions = forloop.counter %
% endif %


% endfor %

<p>Total entries: total_submissions </p>


When I did this, the total_submissions variable did not increment. What am I missing?

% set total_submissions = forloop.counter %

2 Answers
2

Django really discourages changing of data within templates, and they make it pretty hard to do. What you are trying to do won't work in a template.

In your template code, where is submission_list coming from? If you want to iterate over all the submissions in each contest in your contest_list, you would want to do something like:

submission_list

contest_list

% for submission in contest.submission_set.all %


To get the count, if your list is actually a Queryset (it usually is).. then the easiest way to get a count within the template is to use count on the queryset, eg:

count

contest.submission_set.all.count


This will result in another database query, but if you aren't worried about a super high volume of visitors it should be no problem.

As I understand, you want to increase a contest's submission count when a new submission is added. To do that, you'll need

submission_count

Submission

submission_count

Submission

To do (1), you can override the save() method of Submission:

save()

Submission

def save(self, *args, **kwargs):
if self.id is None: # To make sure this is an INSERT, not an UPDATE
contest = Contest.objects.select_for_update().get(id=self.contest.id)
contest.submission_count += 1
contest.save()
super().save(*args, **kwargs) # Call the "real" save() method.


To do (2), you can override the delete() method:

delete()

def delete(self):
contest = Contest.objects.select_for_update().get(id=self.contest.id)
contest.submission_count -= 1
contest.save()
super().delete()


Note that this solution doesn't work if you creating or deleting submissions in bulk. See more at https://docs.djangoproject.com/en/2.1/topics/db/models/#overriding-predefined-model-methods

Personal Opinion: I wouldn't store submission count in db, unless the query to count number of submissions has noticeable impact on performance.

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