如何从 Django 中的两个表计算百分比

Question

I m pretty new to the Django Framework and I am stuck at calculating a percentage. Here m pretty new to the Django Framework and I am stuck at calculating a percentage. Here s the problem:

I have two tables SocialCase and Donation:

class SocialCase(models.Model):

title = models.CharField(max_length=200)
profile = models.ForeignKey(Profile, on_delete=models.CASCADE)
organizer = models.TextField(max_length=200, null=True, blank=False)
description = models.TextField(null=True, blank=False)
created = models.DateTimeField(auto_now_add=True)
profile_image = models.ImageField(upload_to='static/images/')
case_tags = models.ManyToManyField('events.Tag')
target_donation = models.DecimalField(max_digits=5, decimal_places=2, null=True, blank=True)

class Donation(models.Model):

user = models.ForeignKey(User, on_delete=models.CASCADE, null=True, blank=True)
social_case = models.ForeignKey(SocialCase, on_delete=models.CASCADE)
raised = models.DecimalField(max_digits=5, decimal_places=2, null=True, blank=True)

I want to use a @property method to calculate the percentage between raised and target_donation

1. The target_donation field represents the total money needed for this social case. The target_donation will be set when the Social Case is created.
2. The raised field represents the amount of money that has been currently raised. The raised field will be a dynamic value that will increment with each donation

I want to calculate the percentage on the SocialCase model. How do I bring the raised column from Donations model in order to calculate the percentage of each Social Case and output it in the HTML template?

Thank you verry much, and and sorry if this a simple question, I m still a newbie and couldn t find anything in the Django Documentation.

Kind regards, Sergiu

Answer 1

When you define a ForeignKey it creates a "reverse" field on the related object. You can name this using related_name , otherwise it defaults to <modelname>_set (modelname is lowercased). In this case, donation_set

That's probably what you were missing. The code will be something like

@property
def percent_raised(self):

    total = 0.0
    for donation in self.donation_set.all():
        total += float( donation.raised)

    return total / float( self.target_donation) * 100.0

It's more efficient in this case but much less generalizable, to calculate the sum of donations in the DB query using an aggregation function. See the cheat sheet here (third example using Avg , but in this case you'd want Sum not Avg )