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

datefield, django, model

Solution

// 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!

Problem

Original source