How can I combine two or more querysets in a Django view?
I am trying to build the search for a Django site I am building, and in that search, I am searching in three different models. And to get pagination on the search result list, I would like to use a generic object_list view to display the results. But to do that, I have to merge three querysets into one.
How can I do that? I've tried this:
result_list = []
page_list = Page.objects.filter(
Q(title__icontains=cleaned_search_term) |
Q(body__icontains=cleaned_search_term))
article_list = Article.objects.filter(
Q(title__icontains=cleaned_search_term) |
Q(body__icontains=cleaned_search_term) |
Q(tags__icontains=cleaned_search_term))
post_list = Post.objects.filter(
Q(title__icontains=cleaned_search_term) |
Q(body__icontains=cleaned_search_term) |
Q(tags__icontains=cleaned_search_term))
for x in page_list:
result_list.append(x)
for x in article_list:
result_list.append(x)
for x in post_list:
result_list.append(x)
return object_list(
request,
queryset=result_list,
template_object_name='result',
paginate_by=10,
extra_context={
'search_term': search_term},
template_name="search/result_list.html")
But this doesn't work. I get an error when I try to use that list in the generic view. The list is missing the clone attribute.
How can I merge the three lists, page_list, article_list and post_list?
Concatenating the querysets into a list is the simplest approach. If the database will be hit for all querysets anyway (e.g. because the result needs to be sorted), this won't add further cost.
from itertools import chain
result_list = list(chain(page_list, article_list, post_list))
Using itertools.chain is faster than looping each list and appending elements one by one, since itertools is implemented in C. It also consumes less memory than converting each queryset into a list before concatenating.
Now it's possible to sort the resulting list e.g. by date (as requested in hasen j's comment to another answer). The sorted() function conveniently accepts a generator and returns a list:
result_list = sorted(
chain(page_list, article_list, post_list),
key=lambda instance: instance.date_created)
If you're using Python 2.4 or later, you can use attrgetter instead of a lambda. I remember reading about it being faster, but I didn't see a noticeable speed difference for a million item list.
from operator import attrgetter
result_list = sorted(
chain(page_list, article_list, post_list),
key=attrgetter('date_created'))
Try this:
matches = pages | articles | posts
It retains all the functions of the querysets which is nice if you want to order_by or similar.
Please note: this doesn't work on querysets from two different models.
Related, for mixing querysets from the same model, or for similar fields from a few models, starting with Django 1.11 a QuerySet.union() method is also available:
union()union(*other_qs, all=False)New in Django 1.11. Uses SQL’s UNION operator to combine the results of two or more QuerySets. For example:
>>> qs1.union(qs2, qs3)The UNION operator selects only distinct values by default. To allow duplicate values, use the all=True argument.
union(), intersection(), and difference() return model instances of the type of the first QuerySet even if the arguments are QuerySets of other models. Passing different models works as long as the SELECT list is the same in all QuerySets (at least the types, the names don’t matter as long as the types in the same order).
In addition, only LIMIT, OFFSET, and ORDER BY (i.e. slicing and order_by()) are allowed on the resulting QuerySet. Further, databases place restrictions on what operations are allowed in the combined queries. For example, most databases don’t allow LIMIT or OFFSET in the combined queries.
You can use the QuerySetChain class below. When using it with Django's paginator, it should only hit the database with COUNT(*) queries for all querysets and SELECT() queries only for those querysets whose records are displayed on the current page.
Note that you need to specify template_name= if using a QuerySetChain with generic views, even if the chained querysets all use the same model.
from itertools import islice, chain
class QuerySetChain(object):
"""
Chains multiple subquerysets (possibly of different models) and behaves as
one queryset. Supports minimal methods needed for use with
django.core.paginator.
"""
def __init__(self, *subquerysets):
self.querysets = subquerysets
def count(self):
"""
Performs a .count() for all subquerysets and returns the number of
records as an integer.
"""
return sum(qs.count() for qs in self.querysets)
def _clone(self):
"Returns a clone of this queryset chain"
return self.__class__(*self.querysets)
def _all(self):
"Iterates records in all subquerysets"
return chain(*self.querysets)
def __getitem__(self, ndx):
"""
Retrieves an item or slice from the chained set of results from all
subquerysets.
"""
if type(ndx) is slice:
return list(islice(self._all(), ndx.start, ndx.stop, ndx.step or 1))
else:
return islice(self._all(), ndx, ndx+1).next()
In your example, the usage would be:
pages = Page.objects.filter(Q(title__icontains=cleaned_search_term) |
Q(body__icontains=cleaned_search_term))
articles = Article.objects.filter(Q(title__icontains=cleaned_search_term) |
Q(body__icontains=cleaned_search_term) |
Q(tags__icontains=cleaned_search_term))
posts = Post.objects.filter(Q(title__icontains=cleaned_search_term) |
Q(body__icontains=cleaned_search_term) |
Q(tags__icontains=cleaned_search_term))
matches = QuerySetChain(pages, articles, posts)
Then use matches with the paginator like you used result_list in your example.
The itertools module was introduced in Python 2.3, so it should be available in all Python versions Django runs on.