Add a custom admin page for a model and link to it from the detail page

admin.py

from functools import update_wrapper
from django.contrib import admin
from django.contrib.admin import ModelAdmin
from django.contrib.admin.templatetags.admin_urls import add_preserved_filters
from django.core.exceptions import PermissionDenied
from django.shortcuts import render

from myapp.models import Widget
from myapp.forms import ManageWidgetForm


class WidgetAdmin(ModelAdmin):
    change_form_template = 'myapp/change_form.html'
    manage_view_template = 'myapp/manage_view.html'

    def get_urls(self):
        from django.conf.urls import patterns, url

        def wrap(view):
            def wrapper(*args, **kwargs):
                return self.admin_site.admin_view(view)(*args, **kwargs)
            return update_wrapper(wrapper, view)

        info = self.model._meta.app_label, self.model._meta.model_name

        urls = patterns('',
            url(r'^(.+)/manage/$',
                wrap(self.manage_view),
                name='%s_%s_manage' % info),
        )

        super_urls = super(WidgetAdmin, self).get_urls()

        return urls + super_urls

    def manage_view(self, request, id, form_url='', extra_context=None):
        opts = Widget._meta
        form = ManageWidgetForm()
        obj = Widget.objects.get(pk=id)

        if not self.has_change_permission(request, obj):
            raise PermissionDenied

        # do cool management stuff here

        preserved_filters = self.get_preserved_filters(request)
        form_url = add_preserved_filters({'preserved_filters': preserved_filters, 'opts': opts}, form_url)

        context = {
            'title': 'Manage %s' % obj,
            'has_change_permission': self.has_change_permission(request, obj),
            'form_url': form_url,
            'opts': opts,
            'errors': form.errors,
            'app_label': opts.app_label,
            'original': obj,
        }
        context.update(extra_context or {})

        return render(request, self.manage_view_template, context)
admin.site.register(Widget, WidgetAdmin)

change_form.html

{% extends "admin/change_form.html" %}
{% load i18n admin_urls admin_static admin_modify %}

{% block object-tools-items %}
<li>
    {% url opts|admin_urlname:'manage' original.pk|admin_urlquote as manage_url %}
    <a href="{% add_preserved_filters manage_url %}" class="historylink">{% trans "Manage" %}</a>
</li>
{% block.super %}
{% endblock %}

manage_form.html

{% extends "admin/base_site.html" %}
{% load i18n admin_urls admin_static admin_modify %}
{% load humanize tz %}

{% block breadcrumbs %}
<div class="breadcrumbs">
<a href="{% url 'admin:index' %}">{% trans 'Home' %}</a>
&rsaquo; <a href="{% url 'admin:app_list' app_label=opts.app_label %}">{{ app_label|capfirst|escape }}</a>
&rsaquo; {% if has_change_permission %}<a href="{% url opts|admin_urlname:'changelist' %}">{{ opts.verbose_name_plural|capfirst }}</a>{% else %}{{ opts.verbose_name_plural|capfirst }}{% endif %}
&rsaquo; {% trans 'Manage' %} {{ original|truncatewords:"18" }}
</div>
{% endblock %}

{% block content %}
{% timezone "America/Los_Angeles" %}
<div>
  <form action="{{ form_url }}" method="post">
    {% csrf_token %}
    <!-- do cool form things -->
  </form>
</div>
{% endtimezone %}
{% endblock %}