I have a Django application where users can setup stores. I recently added functionality to support opening hours following the advice on this thread - Business Opening hours in Django. My model looks like this:
class LocationHours(SafeDeleteModel):
location = models.ForeignKey(Location, related_name="hours", on_delete=models.CASCADE)
weekday = models.IntegerField(choices=WEEKDAYS, blank=False, null=False)
start_time = models.TimeField(blank=False, null=False, help_text="Opening time (00:00 format)")
end_time = models.TimeField(blank=False, null=False, help_text="Closing time (00:00 format)")
class Meta:
ordering = ('weekday', 'start_time')
unique_together = ('location', 'weekday', 'start_time', 'end_time')
verbose_name_plural = "Location hours"
Process goes like this - these times are entered in a form by the end user and thus assumed to be localtime, most datetimes/times being used in my application are in UTC. I need to compare the two often so originally, I thought I could figure out the timezone of each location object, then whenever I compare something to these OpeningHours, I can just use the given time and the tz (on the associated Location object) to calculate the time in UTC and then it's just a regular datetime comparison.
I wrote the following function to try and fix this:
def is_dt_within_location_hours(location, dt):
# see if time falls within hours
hours = location.hours
if hours.count() > 0:
for hour in hours.all():
dt = dt.astimezone(hour.location.timezone)
if dt.weekday() == hour.weekday:
if hour.start_time < dt.time() < hour.end_time:
return True
return False
else: # this location has no hours
return True
I thought this worked however has some issues.
Primary issue is this - when the Location objects are originally made or edited, I look up the timezone it's in (using the timezonefinder package) and store that in the Location object (using a TimeZoneField) at that time. This is to say, it will not auto update for DST or anything like that as far as I know. I could figure out the timezone everytime I call the above function however I call said function A LOT such that resource wise I'd like to say this is borderline not an option.
I imagine I could find a way to figure out the localtime at the moment they create an OpeningHours object and that way I could just convert to UTC and save it then but I don't know a good way to do that.
I'm thinking now I may need to scrap my entire solution and start from scratch but any advice is really helpful I've been struggling with this for a while.
You're doing it the right way.
You're worried about the timezone offset changing (as with DST) in between the time you record the Location and when you do the computation. But a timezone (represented by a name like "America/Chicago") isn't just an offset, it's a set of rules for computing the local time at any point in history. So it will do the right thing regardless of when you happened to record the timezone name.
A few other notes on the code you posted:
You probably want to make LocationHours unique on just location and weekeday, unless you're purposely trying to allow multiple opening hours for the same location on the same weekday.
Your is_dt_within_location_hours() is fairly inefficient. You don't need to fetch all the LocationHours objects and re-compute the weekday each time. Instead, first compute the local time, then filter location.hours to only include the LocationHours objects on that weekday.