Django annotate with frequency - mysql

Question

(django 3.2.12, python 3.9.3, MySQL 8.0.28)
Imagine models like the following:
class User(models.Model):
email = models.EmailField(...)
created_datetime = models.DatetimeField(...)
class UserLog(models.Model):
created_datetime = models.DatetimeField(...)
user = models.ForeignKey('user.User' ...)
login = models.BooleanField('Log in' ..)
And the following query, destined to annotate each user in the queryset with the frequency of their logs(when log.login=True):
users = User.objects.filter(
Q(...)
).annotate(
login_count=Count('userlog', filter=Q(userlog__login=True)),
login_duration_over=Now() - F('created_datetime'),
login_frequency=ExpressionWrapper(
F('login_duration_over') / F('login_count'),
output_field=models.DurationField()
),
)
This results in a SQL error:
(1064, "You have an error in your SQL syntax;)
The generated SQL (fragment for login_frequency) looks like this:
(
INTERVAL TIMESTAMPDIFF(
MICROSECOND,
`user_user`.`created_datetime`,
CURRENT_TIMESTAMP
) MICROSECOND / (
COUNT(
CASE WHEN `user_userlog`.`login` THEN `user_userlog`.`id` ELSE NULL END
)
)
) AS `login_frequency`,
and MySQL does not seem to like it. A similar code works on SQLlite and, I am told on PG.
What is wrong with the ExpressionWrapper on MySQL, any idea?

Answer 1

Found a workaround:
users = User.objects.filter(
Q(...)
).annotate(
login_count=Count('userlog', filter=Q(userlog__login=True)),
login_duration_over=Now() - F('created_datetime'),
login_frequency=Cast(
ExpressionWrapper(
Cast(F('login_duration_over'), output_field=models.BigIntegerField()) / F('login_count'),
output_field=models.BigIntegerField()
),
output_field=models.DurationField()
)
)
this forces the DIVIDE operation to be performed db-side on bigints and once that is done, cast it back to a timedelta.
MySQL stopped screaming and the results are correct.
Even though that work, this feels ugly. Could there not be a better way?