Flag Field Type Feature Request

[Paul Kenjora]

Hi,

  I'd like to propose a new field type I've used on several projects.  Its a list of boolean flags all in one field.  It replaces the need for many Boolean fields and has the same basic DB lookup capabilities.  This is fully Admin compatible and Database agnostic.

  Here is the full code, I'm sure it could use some minor tweaks.

from django.db import models

from django.forms import MultipleChoiceField, CheckboxSelectMultiple

class FlagField(models.Field):

  description = "A set of flags."

  __metaclass__ = models.SubfieldBase

  def __init__(self, flags, *args, **kwargs):

    self.flags = flags

    super(FlagField, self).__init__(*args, **kwargs)

  def db_type(self, connection):

    return 'char(%s)' % len(self.flags)

  def to_python(self, value):

    if isinstance(value, list) or isinstance(value, set):

      flagged = set(value)

    else:

      flagged = set()

      for i,c in enumerate(value):

        if c == '1' and i < len(self.flags): flagged.add(self.flags[i])

    return flagged

  def get_db_prep_value(self, value, connection, prepared=False):

    return ''.join(map(lambda flag: '1' if flag in value else '0', self.flags))

  def formfield(self, **kwargs):

    defaults = {'form_class': MultipleChoiceField, 'widget':CheckboxSelectMultiple, 'choices':map(lambda f: (f, f), self.flags)}

    defaults.update(kwargs)

    return super(FlagField, self).formfield(**defaults)

Regards,                                                                                  

--
- Paul Kenjora
- 602-214-7285

[Michael Manfre]

Storing bits as a string is not an efficient use of space and is slower to query. Any reason why you are not using the bit datatype with a length? E.g. 'bit(%s)' % len(self.flags)

Regards,

Michael Manfre

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[Russell Keith-Magee]

Agreed - if we were going to implement this, we'd use a bit string implementation, not push it into a char field.

Yours,

Russ Magee %-)

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[Sergiy Kuzmenko]

I often found this handy too. Here's my implementation on bitbucket[1]. There are tests[2] and documentation[3]. It's something I wanted propose to Django core as well but never found time to do so. :)

In my implementation I chose to assign numeric values explicitly:

SetField(choices=((1,"pizza"),(2,"pasta"),(4,"seafood")))

instead of:

SetField(choices=("pizza","pasta","seafood"))

This is for two reasons:

1) It makes it compatible with Django's standard implementation of choices.

2) Implicit index assignment is bug prone if the order of choices changes.

[1] http://goo.gl/16t2h7

[2] http://goo.gl/C75KTq

[3] http://goo.gl/cs0J1U

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