from django.apps import apps
from django.contrib.contenttypes.models import ContentType
from django.contrib.sites.shortcuts import get_current_site
from django.core.exceptions import ObjectDoesNotExist
from django.http import Http404, HttpResponseRedirect
from django.utils.translation import gettext as _


def shortcut(request, content_type_id, object_id):
    """
    Redirect to an object's page based on a content-type ID and an object ID.
    """
    # Look up the object, making sure it's got a get_absolute_url() function.
    try:
        content_type = ContentType.objects.get(pk=content_type_id)
        if not content_type.model_class():
            raise Http404(
                _("Content type %(ct_id)s object has no associated model") %
                {'ct_id': content_type_id}
            )
        obj = content_type.get_object_for_this_type(pk=object_id)
    except (ObjectDoesNotExist, ValueError):
        raise Http404(
            _('Content type %(ct_id)s object %(obj_id)s doesn’t exist') %
            {'ct_id': content_type_id, 'obj_id': object_id}
        )

    try:
        get_absolute_url = obj.get_absolute_url
    except AttributeError:
        raise Http404(
            _('%(ct_name)s objects don’t have a get_absolute_url() method') %
            {'ct_name': content_type.name}
        )
    absurl = get_absolute_url()

    # Try to figure out the object's domain, so we can do a cross-site redirect
    # if necessary.

    # If the object actually defines a domain, we're done.
    if absurl.startswith(('http://', 'https://', '//')):
        return HttpResponseRedirect(absurl)

    # Otherwise, we need to introspect the object's relationships for a
    # relation to the Site object
    try:
        object_domain = get_current_site(request).domain
    except ObjectDoesNotExist:
        object_domain = None

    if apps.is_installed('django.contrib.sites'):
        Site = apps.get_model('sites.Site')
        opts = obj._meta

        for field in opts.many_to_many:
            # Look for a many-to-many relationship to Site.
            if field.remote_field.model is Site:
                site_qs = getattr(obj, field.name).all()
                if object_domain and site_qs.filter(domain=object_domain).exists():
                    # The current site's domain matches a site attached to the
                    # object.
                    break
                # Caveat: In the case of multiple related Sites, this just
                # selects the *first* one, which is arbitrary.
                site = site_qs.first()
                if site:
                    object_domain = site.domain
                    break
        else:
            # No many-to-many relationship to Site found. Look for a
            # many-to-one relationship to Site.
            for field in obj._meta.fields:
                if field.remote_field and field.remote_field.model is Site:
                    try:
                        site = getattr(obj, field.name)
                    except Site.DoesNotExist:
                        continue
                    if site is not None:
                        object_domain = site.domain
                        break

    # If all that malarkey found an object domain, use it. Otherwise, fall back
    # to whatever get_absolute_url() returned.
    if object_domain is not None:
        protocol = request.scheme
        return HttpResponseRedirect('%s://%s%s' % (protocol, object_domain, absurl))
    else:
        return HttpResponseRedirect(absurl)
