Representing a multi-select field for weekdays in a Django model

Question

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?

Without further information (size of dataset, mostly read vs mostly write, etc.) going with bitfields feels like the dreaded Premature Optimization. — Peter Rowell, Sep 08 '10 at 04:48
I will append additional information to the question. Thank you Mr. Rowell. — Belmin Fernandez, Sep 08 '10 at 11:41
Have you considered adding a many to many relationship between a `Weekday` model and the model in question? I know this is a bit of an overkill considering that week days are fixed in number, but it would make filtering very straightforward. — Manoj Govindan, Sep 08 '10 at 11:47
Although the filtering would be straightforward, seems particularly inefficient to me because of static data (weekday names) stored in a db table. However, for the time being, I might just stick with that since it's a simple implementation. — Belmin Fernandez, Sep 08 '10 at 13:42

score 16 · Accepted Answer · answered Nov 16 '11 at 13:51

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.

This answer http://stackoverflow.com/questions/19645227/django-create-multiselect-checkbox-input broaches that subject regarding teh widget — Yablargo, Mar 04 '14 at 22:37
Just want to say thanks again, I was able to stumble through building a widget against your class, and it works like a champ. — Yablargo, Mar 05 '14 at 16:18
@Yablargo and author: Thanks a lot! I also was able to make a widget against your class, but I'm stuck on making the reverse function (from (string)value (let's say "12") to datadict {(4, "wednesday"), (8, "thursday")} to show (and edit) initial data. Could you please help here: http://stackoverflow.com/q/25575951/3849359? Thanks a million! — gabn88, Sep 08 '14 at 11:06
I've got the source code in the office for my working version, I'll dig it up monday — Yablargo, Sep 13 '14 at 02:39

Representing a multi-select field for weekdays in a Django model

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