unsupported operand type(s) for -: 'DateField' and 'DateField'

user_14350333 0
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I am working on creating a contract model with Django and I came cross on how to get the time duration from the start_date to the end_date??

class Contract(models.Model):
    name = models.CharField(max_length=100)
    price = models.IntegerField(max_length=10)
    start_date = models.DateField(auto_now_add=True)
    end_date = models.DateField(auto_now_add=True)
    duration = models.IntegerField(end_date - start_date) # how get the duration by hours
    created_at = models.DateTimeField(auto_now_add=True)
    def __str__(self):
        return self.name
date-formatting model django-models django
2 Answers

If you want to calculate a field in your model one good approach is to do that in an overridden save method.

Substracting one datetime from another results in a timedelta object. I am converting this here into seconds, assuming that is what you wanted.

class Contract(models.Model):
    ...
    duration = models.IntegerField()
    ....

    def save(self, *args, **kwargs):
        self.duration = (self.end_date - self.start_date).total_seconds()
        super().save(*args, **kwargs)
user_43791510

I would advise not to store this in a field: several ORM queries like for example .update(…) [Django-doc] circumvent the .save(…) method [Django-doc], so that means that there are ways to update the start_date, or end_date without the duration being updated properly.

You can add a property that will simply determine the duration when necessary, indeed:

from datetime import timedelta

class Contract(models.Model):
    name = models.CharField(max_length=100)
    price = models.IntegerField(max_length=10)
    start_date = models.DateField(auto_now_add=True)
    end_date = models.DateField(auto_now_add=True)
    created_at = models.DateTimeField(auto_now_add=True)

    @property
    def duration(self):
        return (self.end_date - self.start_date) // timedelta(hours=1)

    def __str__(self):
        return self.name
user_675790
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