A modification of Django's admin widget RelatedFieldWidgetWrapper, which adds the handy + to select boxes when picking a related model. This one doesn't assume the user's picking a related model instance--it simply lets the user pick a model instance or add a new one if needed. This is useful on forms (such as the example in forms.py) where you …

forms.py

"""
An example form using AddAnotherWidgetWrapper.
"""
from django import forms

from .models import Owner
from .widgets import AddAnotherWidgetWrapper


class PickOwnerForm(forms.Form):
    owner = forms.ModelChoiceField(
        queryset=Owner.objects.all().order_by('name'),
        widget=AddAnotherWidgetWrapper(
            forms.Select(),
            Owner,
        )
    )

pick_owner.html

{% extends "admin/base_site.html" %}

{% comment %}
An example template that renders a form where an AddAnotherWidgetWrapper is present.
{% endcomment %}

{% load i18n admin_static admin_modify %}
{% load admin_urls %}

{% block extrahead %}
{{ block.super }}
<script src="{{ STATIC_URL }}admin/js/admin/RelatedObjectLookups.js"></script>

{% block bodyclass %}change-list{% endblock %}

{% block content %}
<div>
    Pick an owner or add a new one:
</div>

{{ form.media }}
<form method="post">
    {% csrf_token %}
    {{ form.as_p }}
    <input type="submit" value="{% trans "Picked" %}" />
</form>

{% endblock %}

widgets.py

import copy

from django import forms
from django.contrib.admin.templatetags.admin_static import static
from django.core.urlresolvers import reverse
from django.utils.safestring import mark_safe
from django.utils.translation import ugettext as _


class AddAnotherWidgetWrapper(forms.Widget):
    """
    This class is a wrapper to a given widget to add the add icon for the
    admin interface. Modeled after 
    django.contrib.admin.widgets.RelatedFieldWidgetWrapper
    """
    def __init__(self, widget, model):
        self.is_hidden = widget.is_hidden
        self.needs_multipart_form = widget.needs_multipart_form
        self.attrs = widget.attrs
        self.choices = widget.choices
        self.widget = widget
        self.model = model

    def __deepcopy__(self, memo):
        obj = copy.copy(self)
        obj.widget = copy.deepcopy(self.widget, memo)
        obj.attrs = self.widget.attrs
        memo[id(self)] = obj
        return obj

    @property
    def media(self):
        return self.widget.media

    def render(self, name, value, *args, **kwargs):
        model = self.model
        info = (model._meta.app_label, model._meta.object_name.lower())
        self.widget.choices = self.choices
        output = [self.widget.render(name, value, *args, **kwargs)]
        related_url = reverse('admin:%s_%s_add' % info)
        output.append(('<a href="%s" class="add-another" id="add_id_%s" ' +
                      'onclick="return showAddAnotherPopup(this);"> ')
                      % (related_url, name))
        output.append('<img src="%s" width="10" height="10" alt="%s"/></a>'
                      % (static('admin/img/icon_addlink.gif'),
                         _('Add Another')))
        return mark_safe(''.join(output))

    def build_attrs(self, extra_attrs=None, **kwargs):
        "Helper function for building an attribute dictionary."
        self.attrs = self.widget.build_attrs(extra_attrs=None, **kwargs)
        return self.attrs

    def value_from_datadict(self, data, files, name):
        return self.widget.value_from_datadict(data, files, name)

    def _has_changed(self, initial, data):
        return self.widget._has_changed(initial, data)

    def id_for_label(self, id_):
        return self.widget.id_for_label(id_)

Hi, I want to use ForeignKeyRawIdWidget with my custom model, due to pop-up window.

class PickForm(forms.Form):
    _selected_action = forms.CharField(widget=forms.MultipleHiddenInput)
    my_id = forms.ModelChoiceField(
        MyModel.objects.all(),
        widget=ForeignKeyRawIdWidget(
            rel=ManyToOneRel('self', admin.site),
            admin_site=admin.site
        )
    )

We are still using LTS django 1.4 and nothing but the plain input is displayed. There is no icon for selecting the choice that I want.

Have you got any suggestions?

@niorko did you manage to solve your problem ?