TypeError：+ 不支持的操作数类型：“DateField”和“datetime.timedelta”

Question

In my model there are start_date_time , end_date_time , period and period_type fields, where end_date_time must be equal to sum of start_date_time and timedelta. And in my case timedelta's day is equal to period and period_type is one of days/months/years/. Here are my codes:

models.py :

class PeriodTypeEnum(models.IntegerChoices):
    Years = 'years'
    Months = 'months'
    Days = 'days'

class Policy(models.Model):
    policy_type = models.ForeignKey('PolicyType', on_delete=models.CASCADE)
    blank = models.OneToOneField('blank_app.Blank', on_delete=models.CASCADE)
    period = models.IntegerField()
    period_type = models.IntegerField(choices=PeriodTypeEnum.choices)
    start_date_time = models.DateField(blank=True, null=True)
    end_date_time = models.DateField(default = start_date_time + timedelta(days=1), blank=True, null=True)

PS: in end_date_time field, timedelta(days + x) - x must be equal to period .

Answer 1

Django doesn't support dependencies like that in default .

The easiest thing to do here is to declare

end_date_time = models.DateField(blank=True, null=True)

and add a

def clean(self):
    if not self.end_date_time:
         # (or do something with `self.period`?)
         self.end_date_time = self.start_date_time + timedelta(days=1)

to the model. Remember that .clean() is only called by forms and such, not when you .save() , so if you want that, you'll need to override .save() too.

def save(self, **kwargs):
    self.clean()
    return super().save(**kwargs)

Answer 2

This code will not work, because start_date_time is not a value, but a field that will keep a value.

As was mentioned in previous answer, you can override clean/save methods to set value for end_date_time . Another option is to use pre_save or post_save signals depending on when/how start_date_time is set.