Django ModelMultipleChoiceField object has no attribute to_field_name

Question

I'm trying to create a custom field for a ModelForm. I'm extending from ModelMultipleChoiceField and then overriding render and render_options, however, I keep getting this exception when just trying to import my form:

AttributeError: 'ModelMultipleChoiceField' object has no attribute 'to_field_name'

I'm not sure what I'm missing. I've tried even adding a to_field_name attribute to my new class, but that doesn't help. Here is my code:

class MultiSelect(ModelMultipleChoiceField):
def __init__(self, queryset, cache_choices=False, required=True,
             widget=None, label=None, initial=None, help_text=None, *args, **kwargs):
    super(MultiSelect, self).__init__(queryset, cache_choices, required, widget,
            label, initial, help_text, *args, **kwargs)

def render_options(self, name, choices, selected_choices):
    output = []
    i = 0
    for option_value, option_label in chain(self.choices, choices):
        checked_html = (option_value in selected_choices) and u' checked="checked"' or ''
        class_html = (i % 2 == 0) and u'even' or u'odd'
        output.append('<li class="{0}"><input type="checkbox" name="{1}" value="{2}"{3}/>{4}</li>'
                .format(class_html, name, escape(option_value), checked_html, escape(option_label)))
        i += 1

def render(self, name, value, attrs=None, choices=()):
    if value is None: value = []
    final_attrs = self.build_attrs(attrs, name=name)
    output = [u'<ul class="multiSelect">']
    options = self.render_options(name, choices, value)
    if options:
        output.append(options)
    output.append('</ul>')
    return mark_safe(u'\n'.join(output))


class RoleForm(ModelForm):
    class Meta:
        model = Role
        exclude = ('user_id',)
        widgets = {
            'permissions': MultiSelect(queryset=Permission.objects.all())
        }

Whenever I simply do an from myapp.forms import RoleForm, I get the error above.

Should I be adding something to my class that I'm missing?

Answer 1

You seem to have got confused between fields and widgets. You inherit from ModelMultipleChoiceField, which (as the name implies) is a field, not a widget. But render and render_options are methods on widgets, not fields. And you've used your class in the widgets dictionary.

I suspect you do mean to create a widget. You should inherit from a widget class, probably forms.CheckboxSelectMultiple.

Answer 2

Not sure why it's a problem based on the code you've posted, but to_field_name is an attribute on ModelChoiceField:

class ModelChoiceField(ChoiceField):
    ...

    def __init__(self, queryset, empty_label=u"---------", cache_choices=False,
                 required=True, widget=None, label=None, initial=None,
                 help_text=None, to_field_name=None, *args, **kwargs):
        ...            

        self.to_field_name = to_field_name

However, when ModelMultipleChoiceField subclasses ModelChoiceField, it's __init__ method doesn't accept to_field_name as an keyword argument. It apparently relies on the ModelChoiceField's default behavior of setting self.to_field_name the default of None.

Your subclass should be doing the same, so that part is confusing.