Case WHEN to fetch data from different model when data is not zero

Question

I have a simple model like this:

     class Payments(models.Model):
       dt=models.DateField()
       payer=models.IntegerField(default=0)

If a member pays for himself, payer=0. But sometimes his support group (there are many groups) can pay for him/her and when that happens, the payer holds the ID of the support group. I didn't want to implement a relationship here.

Now I want to list payments made but when payer>0, to get name of the subgroup that actually paid for the member.

            data=Payments.objects.filter(**build_filters['filters']).annotate(support_group_name=Case(When(payer__gt=0, then=Value('support group name goes here'), default=Value(''), output_field=CharField())).values('support_group_name','id','dt')

I already have a function that use from various locations which returns name of a support group provided with id. I tried:

   data=Payments.objects.filter(**build_filters['filters']).annotate(support_group_name=Case(When(payer__gt=0, then=Value(support_name(payer)), default=Value(''), output_field=CharField())).values('support_group_name','id','dt')

But I get payer is undefined. Any idea what I can do about this?

score 0 · Answer 1 · answered Sep 13 '17 at 14:11

Django supports generic relations that allow you to have a "ForeignKey" like interface that can be to any model in your project

class Payment(models.Model):
    dt = models.DateField()
    payer_content_type = models.ForeignKey(ContentType, on_delete=models.CASCADE)
    payer_object_id = models.PositiveIntegerField()
    payer = GenericForeignKey('payer_content_type', 'payer_object_id')

This generic relation can be limited to a certain set of models - see this question

Payments.objects.filter(payer=user)
Payments.objects.filter(payer=support_group)

Case WHEN to fetch data from different model when data is not zero

1 Answers1