Django query on event list: finding most recent events - django

Question

I have a django model that collects events. It looks roughly like this:
class Events(models.Model):
class EventType(models.TextChoices):
OPEN = ...
CLOSE = ...
OTHER = ...
READ = ...
user = models.ForeignKey(User, on_delete=models.CASCADE)
box = models.ForeignKey(Box, on_delete=models.CASCADE)
event_timestamp = models.DateTimeField(default=django.utils.timezone.now)
event_type = models.CharField(max_length=6, choices=EventType.choices)
Events occur when users do things with boxes.
I have four very related queries I want to do, and I'm not at all sure how to do this without resorting to SQL, which usually isn't the right solution with django.
For box B and user U, is the user most recently open or closed for this box?
For Box B, how many users are in an open state?
For Box B, I'd like to plot day by day how many users are open, so I'd like to get a grouped array of users who opened and closed on each day.
I'd like a table over all boxes of how many users are in an open state.
For example, in SQL these would have forms something like this (not tested code, I'm hoping it's easier to use django's query language):
-- 1
SELECT event_type FROM Events
WHERE event_type in ('OPEN', 'CLOSE')
AND user = U
HAVING max(event_timestamp);
-- 2
SELECT count(*) FROM (
SELECT user, event_type FROM Events
WHERE event_type in ('OPEN', 'CLOSE')
GROUP BY user
HAVING max(event_timestamp)
) as T
WHERE event_type = 'OPEN';
-- 3
// A bit complicated any way I slice it due to the
// mapping from datetime to date.
-- 4
SELECT box, count(user) FROM (
SELECT box, user, event_type FROM Events
WHERE event_type in ('OPEN', 'CLOSE')
GROUP BY box, user
HAVING max(event_timestamp)
) as T
WHERE event_type = 'OPEN'
GROUP BY box;
I've explained my four use cases, but I suspect once I understand one of them, the others will follow. Clearly I've just not grokked something from the docs (I hope).
Many thanks for any pointers.

Answer 1

#jma - I am assuming- For Box B, how many events are in an open state? You mentioned Users are in an open state? But state/event_type(Open,Closed) is tied to the Events model.
box = Box.objects.get(name='B') --- assuming you have a name field in Box entity
events = Events.objects.filter(box=box,event_type='open') #queryset
for event in events:
#your logic here
print(event)
3.For box B and user U, is the event most recently open or closed for this box?
Assuming there can be more than one event for a box B with that user U
box = Box.objects.get(name='B') --- assuming you have a name field in Box entity
user = User.objects.get(username='U') -- you can filter based on any field.
Also, this depends on where you want to implement this functionality. If you are using the Django view then you won't need to query the database for the user object. Since the request object has the user attribute.
In django view, user = request.user
events = Events.objects.filter(box=box,user=user)
for event in events:
status = event.event_type
if status=='Open':
#do something
elif status=='Closed':
#do something
else:
#logic here
Also, just a suggestion
Model name should be singular. Event
If you simply name the event_type field type instead of event_type it's better. We are already in the Event model. So, we don't need to name the fields by tagging the model name as a prefix. However, I would stay away from naming a field type. I prefer category or something else. Same with event_timestamp. You can explicitly name this field to let's say to_be_held_on or created_at (based on your use case)