Set operations on QuerySets

True to their name (or rather the latter half of their name), QuerySets support a lot of (mathematical) set operations. For the sake of illustration, consider two QuerySets that contain the user objects:

    >>> q1 = User.objects.filter(username__in=["a", "b", "c"])
    [<User: a>, <User: b>, <User: c>]
    >>> q2 = User.objects.filter(username__in=["c", "d"])
    [<User: c>, <User: d>]

Some set operations that you can perform on them are as follows:

• Union: This combines and removes duplicates. Use q1 | q2 to get [<User: a>, <User: b>, <User: c>, <User: d>].
• Intersection: This finds common items. Use q1 and q2 to get [<User: c>].
• Difference: This removes elements in the second set from the first. There is no logical operator for this. Instead use q1.exclude(pk__in=q2) to get [<User: a>, <User: b>].

The same operations can be done on QuerySets using the Q objects:

from django.db.models import Q 
 
# Union 
>>> User.objects.filter(Q(username__in=["a", "b", "c"]) | Q(username__in=["c", "d"])) 
[<User: a>, <User: b>, <User: c>, <User: d>] 
 
# Intersection 
>>> User.objects.filter(Q(username__in=["a", "b", "c"]) & Q(username__in=["c", "d"])) 
[<User: c>] 
 
# Difference 
>>> User.objects.filter(Q(username__in=["a", "b", "c"]) & ~Q(username__in=["c", "d"])) 
[<User: a>, <User: b>] 

The difference is implemented using & (and) and ~ (negation). The Q objects are very powerful and can be used to build very complex queries.

However, the Set analogy is not perfect. QuerySets, unlike mathematical sets, are ordered. So, they are closer to Python's list data structure in that respect.