How Do i compare Two model fields against each other in django? -

i have model in models.py file.i want to compare "start_date" , "end_date" start_date value never greater end_date or vice-versa.how do validation?

class completion(models.model):      start_date = models.datefield()     end_date = models.datefield()     batch = models.foreignkey(batch)     topic = models.foreignkey(topic) 

i'd start getting head model validation framework. http://docs.djangoproject.com/en/dev/ref/models/instances/#django.db.models.model.clean

it's used modelforms , makes whole lot of sense start using it.

basically, you'd define clean() method in model, put in validation logic, , raise validationerror if fails.

class mymodel(models.model):     def clean(self):         django.core.exceptions import validationerror         if self.start_data > self.end_date:             raise validationerror('start date cannot precede end date')       def save(self, *args, **kwargs):         # can have regular model instance saves use         super(mymodel, self).save(*args, **kwargs) 

the benefit here modelform (which means admin site too call full_clean(), in turn calls model clean() without work.

no need override save_model, you'll usual validation errors @ top of admin form.

finally, it's super convenient. can use anywhere.

try:     my_model.full_clean() except validationerror, e:     # based on errors contained in e.message_dict.     # display them user, or handle them programatically.