my models:

class Course(models.Model):
    ...


class Section(models.Model):
    course = models.ForeignKey(Course, on_delete=models.CASCADE, related_name='sections')
    ...


class SubSection(models.Model):
    section = models.ForeignKey(Section, on_delete=models.CASCADE, related_name='subsections')
    registration_required = models.BooleanField(default=True)

now I want to know if less than 5 subsections of a course are registration_required = False. something like:

def validate(self, attrs):
    course = self.instance
    
    if course.sections.subsections.filter(registration_required=False).count() < 5:
        # do something ...

what is the best way to do that ??