django and select2

django_select2_mixin.py

'''
I have a model for customer intake. One of the fields is for tracking how the customer heard about us. I do
not want an endless proliferation of values, so I would like to use a multi-select. The problem is I also want the
user to add new choices as needed. It turns out this is possible with the Select2 widget:

https://select2.github.io/examples.html#tokenizer  - check out this link to see how to setup your javascript.

It turns out this is a little tricky to implement. Below is a mixin you can use with a form to handle the field.

Also, if you are using bootstrap, you will want: https://fk.github.io/select2-bootstrap-css/
'''
class Intake(TimeStampedModel):
    referral = models.CharField(max_length=1000, blank=True)  # save as json encoded list

    @classmethod
    def get_referral_choices(cls):
        """
        :return: a set of referal choices
        """
        cut_off = datetime.datetime.now() - datetime.timedelta(200)
        all_db_values = cls.objects.filter(created__gt=cut_off).values_list('referral', flat=True)
        choice_set = {'Google', 'Bing'}
        for v in all_db_values:
            if v:
                choices = json.loads(v)
                choice_set.update(choices)
        return choice_set


class ReferralFieldMixin(object):
    def __init__(self, *args, **kwargs):
        super(ReferralFieldMixin, self).__init__(*args, **kwargs)
        self.fields['referral'].widget.attrs.update({'class': 'select2-tokenizer'})
        self.fields['referral'].help_text = 'To create a new choice, type it followed by a space.'

        referral_choices_set = set(models.Intake.get_referral_choices())
        if self.initial and self.initial.get('referral'):
            self.initial['referral'] = json.loads(self.initial['referral'])
            referral_choices_set.update(self.initial['referral'])
        if 'data' in kwargs and kwargs['data'].get('referral'):
            # kwargs['data'] needs to be converted to a normal dict to get the referral field as a list
            referral_choices_set.update(dict(kwargs['data'])['referral'])
        referral_choices = sorted(list(referral_choices_set))
        self.fields['referral'].choices = [[x, x] for x in referral_choices]

    def clean_referral(self):
        referral = map(methodcaller('capitalize'), self.cleaned_data['referral'])
        return json.dumps(referral)
        
        
class IntakeForm(ReferralFieldMixin, forms.ModelForm):
    referral = forms.MultipleChoiceField(required=False)

    class Meta:
        model = models.Intake
        fields = ['referral']