Checking for overlapping TimeField ranges - python-2.7

Question

I have this model:
class Task(models.Model):
class Meta:
unique_together = ("campaign_id", "task_start", "task_end", "task_day")
campaign_id = models.ForeignKey(Campaign, on_delete=models.DO_NOTHING)
playlist_id = models.ForeignKey(PlayList, on_delete=models.DO_NOTHING)
task_id = models.AutoField(primary_key=True, auto_created=True)
task_start = models.TimeField()
task_end = models.TimeField()
task_day = models.TextField()
I need to write a validation test that checks if a newly created task time range overlaps with an existing one in the database.
For example:
A task with and ID 1 already has a starting time at 5:00PM and ends at 5:15PM on a Saturday. A new task cannot be created between the first task's start and end time. Where should I write this test and what is the most efficent way to do this? I also use DjangoRestFramework Serializers.

Answer 1

When you receive the form data from the user, you can:
Check the fields are consistent: user task_start < user task_end, and warn the user if not.
Query (SELECT) the database to retrieve all existing tasks which intercept the user time,
Order the records by task_start (ORDER BY),
Select only records which validate your criterion, a.k.a.:
task_start <= user task_start <= task_end, or,
task_start <= user task_end <= task_end.
warn the user if at least one record is found.
Everything is OK:
Construct a Task instance,
Store it in database.
Return success.
Implementation details:
task_start and task_end could be indexed in your database to improve selection time.
I saw that you also have a task_day field (which is a TEXT).
You should really consider using UTC DATETIME fields instead of TEXT, because you need to compare date AND time (and not only time): consider a task which starts at 23:30 and finish at 00:45 the day after…

Answer 2

This is how I solved it. It's not optimal by far, but I'm limited to python 2.7 and Django 1.11 and I'm also a beginner.
def validate(self, data):
errors = {}
task_start = data.get('task_start')
task_end = data.get('task_end')
time_filter = Q(task_start__range=[task_start, task_end])
| Q(task_end__range=[task_start, task_end])
filter_check = Task.objects.filter(time_filter).exists()
if task_start > task_end:
errors['error'] = u'End time cannot be earlier than start time!'
raise serializers.ValidationError(errors)
elif filter_check:
errors['errors'] = u'Overlapping tasks'
raise serializers.ValidationError(errors)
else:
pass
return data