Django Rest Framework - passing Model data through a function, then posting output in a separate field in the same model

Question

(Django 2.0, Python 3.6, Django Rest Framework 3.8)

I'm trying to fill the calendarydays field in the model below:

Model

class Bookings(models.Model):
    booked_trainer = models.ForeignKey(TrainerProfile, on_delete=models.CASCADE)
    booked_client = models.ForeignKey(ClientProfile, on_delete=models.CASCADE)
    trainer_availability_only = models.ForeignKey(Availability, on_delete=models.CASCADE)
    calendarydays = models.CharField(max_length=300, blank=True, null=True)

    PENDING = 'PENDING'
    CONFIRMED = 'CONFIRMED'
    CANCELED = 'CANCELED'

    STATUS_CHOICES = (
        (PENDING, 'Pending'),
        (CONFIRMED, 'Confirmed'),
        (CANCELED, 'Canceled')
    )


    booked_status = models.CharField(
        max_length = 9,
        choices = STATUS_CHOICES,
        default = 'Pending'
    )

    def __str__(self):
        return str(self.trainer_availability_only)

Now, I have a function that takes values from trainer_availability_only and converts those values to a list of datetime strings, the returned output would look like this:

{'calendarydays': ['2018-07-23 01:00:00', '2018-07-23 02:00:00', '2018-07-23 03:00:00', '2018-07-30 01:00:00', '2018-07-30 02:00:00', '2018-07-30 03:00:00', '2018-08-06 01:00:00', '2018-08-06 02:00:00', '2018-08-06 03:00:00', '2018-08-13 01:00:00', '2018-08-13 02:00:00', '2018-08-13 03:00:00', '2018-08-20 01:00:00', '2018-08-20 02:00:00', '2018-08-20 03:00:00']}

Problem

How can I fill the calendarydays field with the function output for a user to select from a dropdown, and where should I implement this logic (in my view or the serializer)? My main point of confusion is that, because my function depends on data from trainer_availability_only, I don't want to create a separate model/table for this information (as that would seem too repetitive). I also don't fully understand where in my serializers or views I can implement some sort of dropdown for a User to choose a single calendarydays value for (like I would be able to for a ForeignKey or OneToOneField for example).

Details for the other models aren't really relevant to the question, except trainer_availability_only, which basically gives the user a dropdown selection that would look like this:

('Monday','12:00 am - 1:00 am')
('Wednesday','4:00 pm - 5:00 pm')
etc.

Any help is greatly appreciated.

Answer 1

Assuming that you will not be using this API through browsable API and some client will consume these API's, you should give a separate endpoint to fetch the options which can be shown in the dropdown.

Since the calendar days only depends on the selected Availability, the endpoint can be something like /availability/<:id>/calender-days, where <:id> is the id of the selected option. This will be a GET request with a separate view where you can call your function which calculates calendar days from Availability object and return the list of calendar days. This endpoint will be useful in both cases when you create a new Booking object or you update that.

On your point of showing in the browsable API form, browsable API forms don't load dynamic fields, so it will not call your logic to fetch calendar days based on the selection. For the Foriegn key and one-to-one relation, it already loads the queryset of the models while rendering the form and uses those to show the dropdowns. But if your actual use case is to consume API's using browsable API form then you will have to write some custom javascript to load the values.