How to add custom row level processing of a column in Django Model that is used by Viewsets

Here is my Model for eg. :

class Lics(models.Model):
files_id = models.IntegerField(blank=True, null=True)
seqno = models.IntegerField(blank=True, null=True)
expire = models.TextField(blank=True)


the field 'expire' comes with a date. I have a function that does delta from 'now' when the api is called. How and where do I add an extra field in the ViewSet when calling Lics.objects.all() that has is added as a 4th field which shows 'days' = 4 for eg. if license is expiring 4 days from now.

Expected output : [{files_id:1, seqno:2, expire:'4-jan-2019', 'days':4 }]

[{files_id:1, seqno:2, expire:'4-jan-2019', 'days':4 }]

2 Answers
2

I would probably add it as a SerializerMethodField so that it's calculated upon request. In fact, this is very similar to the example in the docs. You want something like the following (NOTE: I have not tested this, it is just a slight modification of the documented example):

SerializerMethodField

from django.utils.timezone import now
from rest_framework import serializers

class LicSerializer(serializers.ModelSerializer):
days = serializers.SerializerMethodField()

class Meta:
model = Lic

def get_days(self, obj):
cur_time = now()
if cur_time > obj.expire:
return 0
return (obj.expire - cur_time).days


@RishiG has a good solution. As an alternative, you could also make a model property.

class Lics(models.Model):
files_id = models.IntegerField(blank=True, null=True)
seqno = models.IntegerField(blank=True, null=True)
expire = models.TextField(blank=True)

@property
def days(self):
// calculation code goes here


Then, you just include "days" in the "fields" argument of your serializer.

class LicSerializer(serializers.ModelSerializer):

class Meta:
model = Lic
fields = ('files_id', 'seqno', 'expire', 'days')


I generally decide on my approach based on WHERE I want to be able to access this data. For example, if I might want to operate directly on the days property of a model instance I put it in the model. If I only care about bubbling this data up in a view, I put it in the serializer.

days

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