How do I use django's Q with django taggit?

Question

I have a Result object that is tagged with "one" and "two". When I try to query for objects tagged "one" and "two", I get nothing back:

q = Result.objects.filter(Q(tags__name="one") & Q(tags__name="two"))
print len(q) 
# prints zero, was expecting 1

Why does it not work with Q? How can I make it work?

Answer 1

The way django-taggit implements tagging is essentially through a ManytoMany relationship. In such cases there is a separate table in the database that holds these relations. It is usually called a "through" or intermediate model as it connects the two models. In the case of django-taggit this is called TaggedItem. So you have the Result model which is your model and you have two models Tag and TaggedItem provided by django-taggit.

When you make a query such as Result.objects.filter(Q(tags__name="one")) it translates to looking up rows in the Result table that have a corresponding row in the TaggedItem table that has a corresponding row in the Tag table that has the name="one".

Trying to match for two tag names would translate to looking up up rows in the Result table that have a corresponding row in the TaggedItem table that has a corresponding row in the Tag table that has both name="one" AND name="two". You obviously never have that as you only have one value in a row, it's either "one" or "two".

These details are hidden away from you in the django-taggit implementation, but this is what happens whenever you have a ManytoMany relationship between objects.

To resolve this you can:

Option 1

Query tag after tag evaluating the results each time, as it is suggested in the answers from others. This might be okay for two tags, but will not be good when you need to look for objects that have 10 tags set on them. Here would be one way to do this that would result in two queries and get you the result:

# get the IDs of the Result objects tagged with "one"
query_1 = Result.objects.filter(tags__name="one").values('id')
# use this in a second query to filter the ID and look for the second tag.
results = Result.objects.filter(pk__in=query_1, tags__name="two")

You could achieve this with a single query so you only have one trip from the app to the database, which would look like this:

# create django subquery - this is not evaluated, but used to construct the final query
subquery = Result.objects.filter(pk=OuterRef('pk'), tags__name="one").values('id')
# perform a combined query using a subquery against the database
results = Result.objects.filter(Exists(subquery), tags__name="two")

This would only make one trip to the database. (Note: filtering on sub-queries requires django 3.0).

But you are still limited to two tags. If you need to check for 10 tags or more, the above is not really workable...

Option 2

Query the relationship table instead directly and aggregate the results in a way that give you the object IDs.

#  django-taggit uses Content Types so we need to pick up the content type from cache
result_content_type = ContentType.objects.get_for_model(Result)
tag_names = ["one", "two"]
tagged_results = (
    TaggedItem.objects.filter(tag__name__in=tag_names, content_type=result_content_type)
        .values('object_id')
        .annotate(occurence=Count('object_id'))
        .filter(occurence=len(tag_names))
        .values_list('object_id', flat=True)
)

TaggedItem is the hidden table in the django-taggit implementation that contains the relationships. The above will query that table and aggregate all the rows that refer either to the "one" or "two" tags, group the results by the ID of the objects and then pick those where the object ID had the number of tags you are looking for.

This is a single query and at the end gets you the IDs of all the objects that have been tagged with both tags. It is also the exact same query regardless if you need 2 tags or 200.

Please review this and let me know if anything needs clarification.

Answer 2

first of all, this three are same:

Result.objects.filter(tags__name="one", tags__name="two")
Result.objects.filter(Q(tags__name="one") & Q(tags__name="two"))
Result.objects.filter(tags__name_in=["one"]).filter(tags__name_in=["two"])

i think the name field is CharField and no record could be equal to "one" and "two" at same time.

in python code the query looks like this(always false, and why you are geting no result):

from random import choice

name = choice(["abtin", "shino"])

if name == "abtin" and name == "shino":

we use Q object for implement OR or complex queries