How to count model objects in Django-CodePudding

I'm trying to make a student management website with Django. Now I want to count how many students are in a single grade and display the number in my website. How can I do that? My model:

from django.db import models
import uuid
from django.core.validators import MaxValueValidator, 
MinValueValidator

# Create your models here.
class Student(models.Model):
    name = models.CharField(max_length=150)
    roll = models.UUIDField(default=uuid.uuid4, unique=True, 
           primary_key=True, editable=False)
    father_name = models.CharField(max_length=150)
    mother_name = models.CharField(max_length=150)
    phone_number = models.IntegerField(validators= 
                   [MaxValueValidator(99999999999)])
    grade = models.ForeignKey('grade', on_delete=models.CASCADE )

    def __str__(self):
        return self.name


class Grade(models.Model):
    grade_name = models.IntegerField(validators= 
                 [MaxValueValidator(10), MinValueValidator(1)])
    id = models.UUIDField(default=uuid.uuid4, unique=True, 
         primary_key=True, editable=False)

    def __str__(self):
        return self.grade

CodePudding user response：

if you what to group each grade without any filtering, by using following code it will group count of each group and returns it as a dict

from django.db.models import Count

Student.objects.all().order_by('id').values('grade__grade_name').annotate(count=Count('id', distinct=True))

if you want to filter a grade, the following code will return the number of students in elementary

Student.objects.filter(grade__grade_name=elementary).count()

CodePudding user response：

You can use filter() then count() :

first_grade_students = Student.objects.filter(grade__grade_name =1).count()

Remember using count()is more efficient than len()

CodePudding user response：

Use django.db.models Count like this :

from django.db.models import Count

q = Grade.objects.annotate(nb_students=Count('student'))

To access to the number of student in a Grade, do:

for grade in q:
    print(grade.nb_students)