How can my django model DateField add 30 days to the provided value?

Question

as the title suggests. I want to add 30 days to the DateField field. This is auto populated on creation of record using auto_now_add=True

Any ideas how to go about doing this?

Thanks

Answer 1

There is no need to implement custom save method.

Also doing this default=datetime.now()+timedelta(days=30) is absolutely wrong! It gets evaluated when you start your instance of django. If you use apache it will probably work, because on some configurations apache revokes your django application on every request, but still you can find you self some day looking through out your code and trying to figure out why this get calculated not as you expect.

The right way of doing this is to pass a callable object to default argument. It can be a datetime.today function or your custom function. Then it gets evaluated every time you request a new default value.

def get_deadline():
    return datetime.today() + timedelta(days=20)

class Bill(models.Model):
    name = models.CharField(max_length=50)
    customer = models.ForeignKey(User, related_name='bills')
    date = models.DateField(default=datetime.today)
    deadline = models.DateField(default=get_deadline)

Answer 2

// Update

The comment under the original post got me thinking. I guess this is the best solution so far:

from datetime import datetime, timedelta

class MyModel(models.Model):
   mydate = models.DateTimeField(default=datetime.now()+timedelta(days=30))

---

// 2. Update

If you want to define a model attribute which holds the amount of days that should be added you are going to need to override the save method. So far I could'nt come up with a simpler way.

Solution:

class MyModel(models.Model):
  mydate = models.DateTimeField(editable=False)
  daysadded = models.IntegerField()

  def save(self):
    from datetime import datetime, timedelta
    d = timedelta(days=self.daysadded)

    if not self.id:
      self.mydate = datetime.now() + d
      super(MyModel, self).save()

---

As becomingGuru already suggested you should override your models save method.

Example:

class MyModel(models.Model):
  mydate = models.DateTimeField(auto_now_add=True)      

  def save(self):
    from datetime import timedelta
    d = timedelta(days=30)

    // only add 30 days if it's the first time the model is saved
    if not self.id:
      // not saving the model before adding the timedelta gave me errors 
      super(MyModel, self).save()

      self.mydate += d

      // final save
      super(MyModel, self).save()

This is not the best way for me since you have to save the model twice. But using auto_now_add requires you to save the model first before a datetime instance for mydate is created.

Another approach which would require only one save:

class MyModel(models.Model):
  mydate = models.DateTimeField(editable=False) // editable=False to hide in admin

  def save(self):
    from datetime import datetime, timedelta
    d = timedelta(days=30)

    // only add 30 days if it's the first time the model is saved
    if not self.id:
      self.mydate = datetime.now() + d
      super(MyModel, self).save()

Hope that helped!

Answer 3

Use Pythons timedelta:

from datetime import timedelta
d = timedelta(days=30)

# object is your current instance of the model
object.yourdatefield += d
# or this because I am not sure whether the previous works
object.yourdatefield = object.yourdatefield + d

object.save()

And from the Django documentation:

DateField
A date, represented in Python by a datetime.date instance.

If you want to add 30 days on creation of the object, forget about auto_now_add=True and do as becomingGuru suggests. Information about overriding save() can also be found in the Django documentation.

Answer 4

Override the save on the model and while saving, check if pk is populated.

>>> from datetime import datetime
>>> from datetime import timedelta
>>> cur_date = datetime.now()
>>> cur_date
datetime.datetime(2010, 2, 4, 5, 0, 24, 437405)
>>> cur_date+timedelta(days=30)
datetime.datetime(2010, 3, 6, 5, 0, 24, 437405)

Answer 5

doc for timedelta: https://docs.python.org/2/library/datetime.html

def make_timedelta(seconds):
        return timedelta(days=seconds // 86399, seconds=seconds % 86399)