传递几个参数以渲染模板

Question

First of all, let me excuse for my English - it's not my native language.

I have following models:

class Student(models.Model):
    student_id = models.IntegerField(primary_key=True)
    student_name = models.CharField(max_length=255, default='John Doe')
    dob = models.DateField(max_length=8)

class Group(models.Model):
    group_name = models.CharField(max_length=40)
    monitor = models.ForeignKey(Student)

class Student_Group(models.Model):
    student_id = models.ForeignKey(Student)
    group_name = models.ForeignKey(Group)

I need to render groups of students, its monitors and amount of students in each group. Making first two tasks is not a problem:

views.py:

def group_list(request):
    groups = Group.objects.all()
    return render(request, 'groups/group_list.html', {'groups': groups})

groups.html

{% for group in groups %}
    <p>{{ group.group_name }}</p>
    <p>{{ group.monitor }}</p>
{% endfor %}

But when it comes to rendering amount of students for each group, I'm getting stuck.

Following SQL lets to count amount of students in given group

select count(*) from students_student
join students_student_group
on students_student.student_id = students_student_group.student_id_id
where students_student_group.group_name_id = "Mega nerds"

Questions are:

1. How to get amount of students for each group using Django ORM instead, so the template will render following info:

   • Group name: Mega nerds
   • Amount of students: 8
   • Monitor: John Doe

   ...

   • Group name: Nice guys
   • Amount of students: 11
   • Monitor: John Appleseed
2. How to pass data regarding amount of students to corresponding group.

Thanks.

Update

According to @Gocht advice, I used ManyToManyField, so my models.py now looks as

class Student(models.Model):
    student_id = models.IntegerField(primary_key=True)
    student_name = models.CharField(max_length=255, default='Василий Пупкин')
    dob = models.DateField(max_length=8)

    def __str__(self):
        return self.student_name


class Group(models.Model):
    group_name = models.CharField(max_length=40, primary_key=True)
    monitor = models.ForeignKey(Student)
    students = models.ManyToManyField(Student, related_name='students')

also, as suggested, I've added decorator to Group class:

@property
def get_students_qty(self):
    return self.students.all().count()

so now I can get number of students in each group, like so:

{% for group in groups %}
    <p>{{ group.group_name }}</p>
    <p>{{ group.monitor }}</p>
    <p>{{ group.get_students_qty }}</p>
{% endfor %}

But I still wondering - is it possible to get number of students in group without using decorator? After all, Group class has students field...

Answer 1

You could get the number of students in a group like this:

group = ...  # get a group

n_students = Student_Group.objects.filter(group=group).count()

Then since every Student_Group object has one student, n_students will contain the number of student in the given group.

To send this number to your template you can add it in your context:

def group_list(request):
    groups = Group.objects.all()
    return render(request, 'groups/group_list.html', {'groups': groups, 'n_students': n_students})

You could also see docs for ManyToMany relationships, that could be helpful here.

EDIT

Take some time to check Python's naming conventions ; your Student_Group should be StudentGroup .

You can create a method in your model to return the number of students in a group:

# models.py
class Group(models.Model):

    # fields

    @property
    def get_students_qty(self):
        return self.student_group_set.all().count()
        # Try with self.studentgroup_set.all().count() if the line
        # above does not work

then in your template:

{% for group in groups %}
    <p>{{ group.group_name }}</p>
    <p>{{ group.monitor }}</p>
    <p>{{ group.get_students_qty }}</p>
{% endfor %}