Complex lookups with Q objects in Django

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One of the great offers of Django is the high-level database API, which lets us manipulate our data using an object oriented approach.

For instance given the following model:

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from django.db import models
 
class Grade(models.Model):
    student_code = models.CharField(u'Student Code', max_length=30)
    course_code = models.CharField(u'Course code', max_length=30)

What if we want to obtain all the grades for some combinations of student and courses. We might be tempted to use a *filter(**kwargs)* method with the in lookup:

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from grades.models import Grade
 
student_codes = ['456870','433495','104248','720400']
course_codes = ['474009','431934','447595','789614']
grades = Grade.objects.filter(student_code__in=student_codes, 
                    course_codes__in=course_codes)

However, this is not ideal since we will end up with grades from different combinations of student/course. This where Q objects come for the rescue. Q objects encapsulate conditions and can be merged together with different logical operators, such as OR and AND. Therefore, we could rewrite our previous approach as follows:

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from grades.models import Grade
from django.db.models import Q
import operator
from itertools import izip
 
student_codes = ['456870','433495','104248','720400']
course_codes = ['474009','431934','447595','789614']
 
q_objects = (Q(student_code=sc, course_code=cc) for sc, cc in izip(student_codes, course_codes))
 
query = reduce(operator.or_, q_objects)
grades = Grade.objects.filter(query)

The important part here is that we joining together a set of OR statements (our desired combinations) instead of allowing forbidden combinations that we are not interested. This example shows how we can overcome the limitations of filter methods by using Q objects, another great feature of our framework.