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

I had many time the need to optimize some performance issues with Django ORM queries, then many times the [prefetch_related][1] and [select_related][2] 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`:

```python

    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)

```

```python

    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`:

```python

    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:

```python

    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.


[1]: https://docs.djangoproject.com/en/1.10/ref/models/querysets/#prefetch-related
[2]: https://docs.djangoproject.com/en/1.10/ref/models/querysets/#select-related