python - Django model update on fetch -

my model has field should change if it's within date range.

it this:

class election(models.model)     start_date = models.datetimefield(verbose_name = 'start date')     end_date = models.datetimefield(verbose_name = 'end date')     active = models.booleanfield(default=false)      def updateactive(self):         = timezone.now()         if self.start_date < , self.end_date > now:             self.active=true         else:             self.active=false         self.save() 

right now, every time query model, call updateactive() views.py.

so, question is: there way call updateactive() every time fetch election object? or keeping constant updated?

any idea welcome.

the best method not have active field @ in model. main reason when value can generated simple calculation, should not stored in database. second reason booleanfield cannot indexed , queries involving field slow. therefore not lose doing calculation instead of doing field. best way add custom queryset this:

class electionqueryset(models.queryset):     def is_active(self):         return self.filter(start_date__lt=timezone.now()).filter(end_date__gt=timezone.now()) 

now model simple.

class election(models.model): start_date = models.datetimefield(verbose_name = 'start date') end_date = models.datetimefield(verbose_name = 'end date')

objects = electionqueryset.as_manager() 

now model realy simple.

class election(models.model):     start_date = models.datetimefield(verbose_name = 'start date')     end_date = models.datetimefield(verbose_name = 'end date')      objects = electionqueryset.as_manager() 

yes that's all. there no need update database everytime fetch object! can use simple method find out what's active or not

election.objects.is_active() 

the result is_active queryset, , can chain usual

election.objects.is_active().filter(...) 

if want check if election active in template can :

class election(models.model):     def is_active()          if self.start_date < , self.end_date > now:             return true