sitebasedsearch.py

# search_indexes.py
from haystack import indexes
from myapp.models import TheModel


class TheModelSearchIndex(indexes.SearchIndex, indexes.Indexable):
    # The usual fields.
    # Then...
    sites = indexes.MultiValueField()

    def get_model(self):
        return TheModel

    def prepare_sites(self, obj):
        return list(obj.sites.all().values_list('pk', flat=True))


# forms.py
from django.contrib.sites.models import Site
from haystack.forms import SearchForm


class SiteBasedSearchForm(SearchForm):
    def search(self):
        results = super(SiteBasedSearchForm, self).search()
        site = Site.objects.get_current()
        return results.filter(sites=site.pk)


# urls.py
from django.conf.urls.defaults import patterns, url
from haystack.views import SearchView
from wherever.forms import SiteBasedSearchForm

urlpatterns = patterns('',
    # The usual.

    # ...Then...
    url(r'^search/$', SearchView(form_class=SiteBasedSearchForm))
)