[Django]-Simple way for QuerySet union and subtraction in django?

Since Django 1.11, QuerySets have union(), intersection() and difference() methods.

It’s also possible to use & and | infix operators with QuerySets (I could not find a reference to this in the docs, so I guess union() and intersection() is the preferred way to combine two querysets.

qs3 = qs1.union(qs2)         # or qs3 = qs1 | qs2
qs3 = qs1.intersection(qs2)  # or qs3 = qs1 & qs2
qs3 = qs1.difference(qs2)    # no operator for this

You can also use Q() objects which like QuerySets implement | and &, and additionally the invert prefix operator ~.

Neither class implement the xor / symmetric difference infix operator ^.

Subtract a QuerySet from another QuerySet using the same model.

This works – but is probably slowly

queryset_with_hello = Blog.objects.filter(name__icontains='hello')
queryset_without_hello = Blog.objects.exclude(pk__in=queryset_with_hello)

Read the performance considerations in django documentation:

https://docs.djangoproject.com/en/dev/ref/models/querysets/#in

Going back to django’s documentation, you can:

new_query_set = query_set_1 | query_set_2

This works as a logical OR which is actually addition without duplicates. This answers the addition aspect and AFAIK does not hit the db at all!

new_query_set = query_set_1 & query_set_2

This works as a logical AND.

Still missing how to subtract QuerySets. It’s hard for me to believe this has not been dealt with elegantly by the community…

You can use the Q object.

The syntax could be something like this:

added_query_set = YourModel.objects.\
         filter(Q(id__in=old_query_set_1)|Q(id__in=old_query_set_2))

You probably can optimize based on your actual needs and get the amount of db hits down (right now it’s 3), but this should get you started.

qs3 = qs1.union(qs2) # union
qs4 = qs1.difference(qs2) # subtraction

I think for operations as this you need to evalute them. So you can call list() on them and work on them with the common python list operations!