I am using Django. Is there a way to count between different fields in a queryset with the same foreign key??!!

That is, we want to subtract register_date from injection_date. You want to get (2021-1-03) - (2021-10-31) = 3days.

Below is the code I've written so far, how do I fix it?! Please help. [models.py]

class Student(models.Model):
    name = models.CharField(max_length=500, blank=True, null=True)
    ...

class Feedback(models.Model):
    injection_date = models.DateField(blank=True, null=True)
    enroll_date = models.DateField(blank=True, null=True)
    student_id = models.ForeignKey(Student, on_delete=models.SET_NULL, null=True, blank=True)

[views.py]

injection_enroll = Feedback.objects\
          .annotate(enroll_injection=F('enrolL_date') - F('injection_date'))

CodePudding user response：

This will get you all students annotated with their max enroll_date minus their min injection_date. The min/max is for removing duplicate values that may occur for a student

Student.objects.annotate(
    injection_date=Min('feedback__injection_date'),
    enroll_date=Max('feedback__enroll_date')
).annotate(
    enroll_injection=F('enroll_date') - F('injection_date')
)

CodePudding user response：

Try this one:

from django.db.models import IntegerField, F
from django.db.models.functions import Cast, ExtractDay, TruncDate


injection_enroll = Feedback.objects.annotate(
    enroll_injection=Cast(
        ExtractDay(TruncDate(F('enrolL_date')) - TruncDate(F('injection_date'))),
        IntegerField()
    )
)

CodePudding user response：

from django.db.models import F
my_list = todolist.objects.order_by('injection_date').filter(created__gte=F('enroll_date'))

Template:

{% if my_list %}
{% for list in my_list %}
{{ list.text}}
{% endfor %}
{% else %}
<p>there is no list</p>
{% endif %}

In the end I get an empty list, but I know, it is not correct. I have also used the following inside the template (without queryset filter):

{% if list.created|date:'d m y h:i:s' == list.modified|date:'d m y h:i:s' %}

It worked, but I would prefer to see a more elegant solution.