Changing showed form field in ManyToManyFields, Django

forms.py

from django import forms
from django.forms import ModelForm
from links.models import Link, Tag

from django.forms.util import ErrorList


def model_to_dict(instance, fields=None, exclude=None):
    """
    Returns a dict containing the data in ``instance`` suitable for passing as
    a Form's ``initial`` keyword argument.

    ``fields`` is an optional list of field names. If provided, only the named
    fields will be included in the returned dict.

    ``exclude`` is an optional list of field names. If provided, the named
    fields will be excluded from the returned dict, even if they are listed in
    the ``fields`` argument.
    """
    # avoid a circular import
    from django.db.models.fields.related import ManyToManyField
    opts = instance._meta
    data = {}
    for f in opts.fields + opts.many_to_many:
        if not f.editable:
            continue
        if fields and not f.name in fields:
            continue
        if exclude and f.name in exclude:
            continue
        if isinstance(f, ManyToManyField):
            # If the object doesn't have a primry key yet, just use an empty
            # list for its m2m fields. Calling f.value_from_object will raise
            # an exception.
            if instance.pk is None:
                data[f.name] = []
            else:
                data[f.name] = ", ".join([obj.slug for obj in f.value_from_object(instance)])
        else:
            data[f.name] = f.value_from_object(instance)
    return data


class LinkForm(ModelForm):
    tags = forms.ModelMultipleChoiceField(
                                queryset=Tag.objects.all(),
                                widget=forms.Textarea
                              )

    def __init__(self, data=None, files=None, auto_id='id_%s', prefix=None,
                 initial=None, error_class=ErrorList, label_suffix=':',
                 empty_permitted=False, instance=None):
        opts = self._meta
        if instance is None:
            if opts.model is None:
                raise ValueError('ModelForm has no model class specified.')
            # if we didn't get an instance, instantiate a new one
            self.instance = opts.model()
            object_data = {}
        else:
            self.instance = instance
            object_data = model_to_dict(instance, opts.fields, opts.exclude)
        # if initial was provided, it should override the values from instance
        if initial is not None:
            object_data.update(initial)
        # self._validate_unique will be set to True by BaseModelForm.clean().
        # It is False by default so overriding self.clean() and failing to call
        # super will stop validate_unique from being called.
        self._validate_unique = False
        super(ModelForm, self).__init__(data, files, auto_id, prefix, object_data,
                                        error_class, label_suffix, empty_permitted)

    class Meta:
        model = Link
        fields = ('title', 'url', 'description', 'tags')


class UploadFileForm(forms.Form):
    file = forms.FileField()