Mounir Messelmeni
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Accessing prefetched objects inside your model's methods - Django ORM

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Accessing prefetched objects inside your model's methods - Django ORM

Mounir Messelmeni's photo
Mounir Messelmeni
·Feb 6, 2017·

2 min read

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I had many time the need to optimize some performance issues with Django ORM queries, then many times the prefetch_related and select_related were my best tool for this problem. But when calling some business logic which reside in some model's methods which try to calculate or to fetch some related model data the optimization will not have any effect as these method are using filter or some queryset aggregations Sum or Count ...

The solution to this problem is either to redefine the logic to use prefetched data or more clean to make these methods now if there already prefetched objects, that's the best solution in my cases:

A basic models example models.py:


    class Teacher(models.Model):
        name = models.CharField(max_length=255)

        def students_count(self):
            return self.student_set.filter(age__gte=20).count()

    class Student(models.Model):
        name = models.CharField(max_length=255)
        teacher = models.ForeignKey(Teacher)
        age = models.IntegerField(default=20)

    for t in Teacher.objects.all():
        print(teacher.students_count())
    # This will results into multiple DB queries equal to the number of teachers + 1
    # If we have 100 teachers in the DB, this will be causing 101 queries.

Let's use prefetch_related:


    for t in Teacher.objects.all().prefetch_related('student_set'):
        print(teacher.students_count())
    # Our prefetch have no effect as the method students_count is using filter which
    # will ignore the prefetched objects

We have to fix the method students_count to consider checking for prefetched objects:


    class Teacher(models.Model):
        name = models.CharField(max_length=255)

        def students_count(self):
            if hasattr(self, '_prefetched_objects_cache') and 'student' in self._prefetched_objects_cache:
                return len([x for x in self.student_set.all() if x.age >= 20])
            return self.student_set.filter(age__gte=20).count()

This way even calling students_count in a single instance without calling prefetch_related on the queryset will use the filter instead of the prefetched.

 
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