I've been searching for an elegant way to represent a multi-select weekday field (Mon, Tues, Wed...) in a Django model. I was initially thinking of going integer field using bitwise math but I am not sure if this would be the way to go.
This would be a mostly-read field. I would want the Queryset method to be something like Entry.objects.get(weekdays__contains=MONDAY)
Where MONDAY
would be a constant.
Perhaps someone could come up with a better solution? Or maybe someone has done something similar and has some example code they could contribute?
This is an old question, but I thought I would show how it could be done reasonably simply in Django.
Here is a helper class for preparing your choices:
class BitChoices(object):
def __init__(self, choices):
self._choices = []
self._lookup = {}
for index, (key, val) in enumerate(choices):
index = 2**index
self._choices.append((index, val))
self._lookup[key] = index
def __iter__(self):
return iter(self._choices)
def __len__(self):
return len(self._choices)
def __getattr__(self, attr):
try:
return self._lookup[attr]
except KeyError:
raise AttributeError(attr)
def get_selected_keys(self, selection):
""" Return a list of keys for the given selection """
return [ k for k,b in self._lookup.iteritems() if b & selection]
def get_selected_values(self, selection):
""" Return a list of values for the given selection """
return [ v for b,v in self._choices if b & selection]
Define your model with a PositiveIntegerField, and the choices you would like:
WEEKDAYS = BitChoices((('mon', 'Monday'), ('tue', 'Tuesday'), ('wed', 'Wednesday'),
('thu', 'Thursday'), ('fri', 'Friday'), ('sat', 'Saturday'),
('sun', 'Sunday')
))
This means you can access the values like this:
>>> print list(WEEKDAYS)
[(1, 'Monday'), (2, 'Tuesday'), (4, 'Wednesday'), (8, 'Thursday'), (16, 'Friday'), (32, 'Saturday'), (64, 'Sunday')]
>>> print WEEKDAYS.fri
16
>>> print WEEKDAYS.get_selected_values(52)
['Wednesday', 'Friday', 'Saturday']
Now define your model with a PositiveIntegerField
and these choices:
class Entry(models.Model):
weekdays = models.PositiveIntegerField(choices=WEEKDAYS)
And your models are done. For queries, the following does the trick:
Entry.objects.extra(where=["weekdays & %s"], params=[WEEKDAYS.fri])
There may be a way to create a Q()
object subclass that neatly packages queries, so they look like this:
Entry.objects.filter(HasBit('weekdays', WEEKDAYS.fri))
Or even hack at a F()
subclass to create something like this:
Entry.objects.filter(weekdays=HasBit(WEEKDAYS.fri))
But I don't have the time to explore that at the moment. .where
works fine and can be abstracted into a queryset function.
One final consideration is that you might light to make a custom model field that converts the bit mask in the database to a list or set in Python. You could then use a SelectMultiple
widget (or CheckboxSelectMultiple
) to allow the user to select their values in the admin.
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