python - Better way to set default value in django model field -

i implementing field in django model calls following function default value

def get_default_value():     = mymodel.objects.aggregate(max_id=max('id'))     return get_unique_value_based_on_num(a['max_id'] or 0)  class mymodel:     default_value_field = charfield(default=get_default_value) 

although may wrong on this, fear implementation may result in race condition.

is there better way ? may use f object or else ?

to avoid race conditions, best letting database handle integrity of table, 1 of things databases made for.

to so, catch integrityerror raised saving model instance , try again different value when fails.

from django.db import integrityerror, models, transaction   def get_default_value():     = mymodel.objects.aggregate(max_id=max('id'))     return get_unique_value_based_on_num(a['max_id'] or 0)  class mymodel(models.model):     # have unicity enforced @ database level unique=true.     default_value_field = models.charfield(max_length=200, unique=true)      def save(self):         if not self.default_value_field:             max_tries = 100  # choose sensible value!             in range(max_tries):                 try:                     self.default_value_field = get_default_value()                      # atomic block rollback transaction in case of integrityerror.                     transaction.atomic():                         super(mymodel, self).save()                         break                 except integrityerror:                     # default_value_field not unique, try again new value.                     continue             else:                 # max tries reached, raise.                 raise integrityerror('could not save model because etc...')         else:             super(mymodel, self).save(*args, **kwargs)