Question

class Session(models.Model):
  tutor = models.ForeignKey(User)
  start_time = models.DateTimeField()
  end_time = models.DateTimeField()

What is the best way to always prevent start_time from overlapping end_time ?

Answer 1

First, you should do this at the form level, see Cleaning and validating fields that depend on each other for instructions on how to do this, it would look like this:

def clean(self):
    cleaned_data = super(YourForm, self).clean()
    if cleaned_data['start_time'] > cleaned_data['end_time']:
        raise forms.ValidationError(_(u'Start time must be before end time'))

Also, you can enforce it at low level in the save method or in a pre_save signal:

from django.db.models import signals

class StartTimeAfterEndTimeException(Exception):
    def __init__(self, model):
        super(StartTimeAfterEndTimeException, self).__init__(
            'Start time %s should not be after end time %s' % (
                model.start_time, model.end_time))

def session_force_end_time(sender, instance, **kwargs):
    if instance.start_time >= instance.end_time:
        raise StartTimeAfterEndTimeException(instance)
signals.pre_save.connect(session_force_end_time, sender=Session)

But that’s pretty low level so it’s not user friendly.