I am trying to write a custom PostgreSQL function in Django that will coerce datetimes to a specified timezone inside of a queryset. My first pass at the db function looks like this:
from django.db.models.expressions import Func
class DateTimeInTimezone(Func):
template="%(expressions)s AT TIME ZONE %(tz_info)s"
This function works in the simple case where I pass a timezone string into the function directly like so:
MyModel.objects.filter(timefield__gte=DateTimeInTimezone(Now(), tz_info='EST'))
However it doesn't work in the more complex case, where the timezone is defined on some field on the model. Consider the following contrived example:
class User(models.Model):
time_zone = models.CharField()
class Meeting(models.Model):
users = models.ManyToManyField(User, related_name='meetings')
start_time = models.DateTimeField() # in UTC
end_time = models.DateTimeField() # in UTC
To answer the question "What users will be in a meeting at 12pm local time today?", I'd need some variation of this queryset:
noon_utc = ...
User.objects.filter(
meetings__start_time__lte=DateTimeInTimezone(noon_utc, tz_info=F('time_zone')),
meetings__end_time__gt=DateTimeInTimezone(noon_utc, tz_info=F('time_zone'))
)
As currently written, however, DateTimeInTimezone will simply inject the string F('time_zone') into the sql rather than resolving the field.
Is it possible to add support for F Expressions to this function? Is there some other approach that I should consider?
A simple solution is possible with parameter arg_joiner:
class DateTimeInTimezone(Func):
function = ''
arg_joiner = ' AT TIME ZONE '
def __init__(self, timestamp, tz_info):
super(DateTimeInTimezone, self).__init__(timestamp, tz_info)
The method __init__ is used only for the purpose of readability with clear names of parameters. Then arity is not important if parameters are declared by __init__.
A oneliner function is useful for fast development if readability is not important:
...filter(
meetings__start_time__lte=Func(noon_utc, tz_info=F('time_zone'), arg_joiner=' AT TIME ZONE ', function=''),
)
Verified:
>>> qs = User.objects.filter(...)
>>> print(str(qs.query))
SELECT ... WHERE ("app_meeting"."start_time" <= ((2017-10-03 08:18:12.663640 AT TIME ZONE "app_user"."time_zone")) AND ...)
Found an acceptable solution. I overrode the as_sql method for function like so, allowing the django internals to resolve the F expression and then separating it back out into a kwarg I could use in a different part of the template.
class DateTimeInTimezone(Func):
'''
Coerce a datetime into a specified timezone
'''
template="%(expressions)s AT TIME ZONE %(tz_info)s"
arity = 2
def as_sql(self, compiler, connection, function=None, template=None, arg_joiner=None, **extra_context):
connection.ops.check_expression_support(self)
sql_parts = []
params = []
for arg in self.source_expressions:
arg_sql, arg_params = compiler.compile(arg)
sql_parts.append(arg_sql)
params.extend(arg_params)
data = self.extra.copy()
data.update(**extra_context)
# Use the first supplied value in this order: the parameter to this
# method, a value supplied in __init__()'s **extra (the value in
# `data`), or the value defined on the class.
if function is not None:
data['function'] = function
else:
data.setdefault('function', self.function)
template = template or data.get('template', self.template)
arg_joiner = arg_joiner or data.get('arg_joiner', self.arg_joiner)
data['expressions'] = data['field'] = arg_joiner.join(sql_parts)
parts = data['expressions'].split(', ')
data['expressions'] = parts[0]
data['tz_info'] = parts[1]
return template % data, params
I added the three lines between the assignment of data['expressions'] and the final return template % data, params. This isn't a great long term solution as the django internals for this method could change in the next version, but it suits my needs for the time being.