如何在 django 中获取 model 对象的计数

Question

Would anyone try to explain to me why I can't get a count of what is the query. I'd like to get count for all the books borrowed by every student

def view_student(request):  
    issues = Student.objects.all()
    for issue in issues:
        borroweds = Issue.objects.filter(borrower_id__student_id = issue.student_id).count()
        print(borroweds)
    return render(request, 'view_student.html', {'borroweds':borroweds})

And the tml code is this.

{% for student in students %}
    <tr>
      <td>{{ forloop.counter }}</td>
      <td>{{ student.student_id }}</td>
      <td>{{ student.name }}</td>
      <td>{{ student.batch }}</td>
      <td>{{ student.semester }}</td>
      <td><a href="{% url 'all_borrowed' student.student_id %}"> {{ borroweds }}</a> </td>
     </tr>
  {% endfor %}

This gives me a list of all students with books borrowed print(borroweds)

while in the template {{ borroweds }} gives me 0 's Am I calling it the wrong way of the code in the views????

class Student(models.Model):
    school = models.ForeignKey(School, on_delete=models.CASCADE)
    name = models.CharField(max_length=200)
student_id = models.CharField(max_length=20, unique=True)

class Issue(models.Model):
    borrower_id = models.ForeignKey(Student,on_delete=models.CASCADE)
    book_id = models.ForeignKey(Books,on_delete=models.CASCADE)
    issue_date = models.DateField(default=datetime.date.today)

Answer 1

You should be able to do something like this with Aggregation: Following relationships backwards

This will not work out of the box, I'm not sure how your models are designed. If you post them I can help you further.

If you customized this to your models it would add the field issue__count to every Student .

from django.db.models import Count

def view_student(request): 
    students = Student.objects.annotate(Count('issue'))
    return render(request, 'view_student.html', {'students': students})

Answer 2

The problem is, every time you loop through the issues, you are re-declaring the borroweds variable. Hence, when the loop ends, you are left with the number of books borrowed by the last person (ie the last issue instance of the issues queryset). That's why you are probably getting a zero.

Answer 3

You're overriding borroweds at each iteration of your for loop. Don't iterate over a Django queryset, you usually don't need to do this.

Instead, use annotate and count.

from django.db.models import Count

def view_student(request):  
    students = Student.objects.annotate(borrowed_books=Count('issue'))
    return render(request, 'view_student.html', {'students':student})

And in your template, you can access it with:

{% for student in students %}
{{ student.borrowed_books }}
{% endfor %}