Tastypie & Django-Polymorphic

I ran into this problem last night, and since Googling for it didn't help, I thought it prudent to post my solution publicly in case anyone else might have a similar problem.

Tastypie is a nifty REST API server app for Django that does a lot of the work for you when all you want to do is share your model data with the world using standardised methods. It's smart enough to do introspection of your models and then use what it finds to make serialisation decisions further down the line. That's how you get from a series of model field definitions to an easy to parse JSON object.

Django-Polymorphic is an amazing piece of software that lets you effortlessly dump polymorphic attributes into your models in an understandable and performant way. With it you can do things like ask for a list of Items and get back a series of EdibleItems and MetalItems.

While both of these technologies are awesome, they don't appear to play nice together. From what I can tell, this is due to how Tastypie does its introspection: at startup time rather than run time. Introspection is done once, on the initial class, and then never again, so polymorphism just won't work.

To fix this, you have one or two options: (a) Break up your API by item type using something like /api/v1/edible-items/ and /api/v1/metal-items/ rather than just /api/v1/items/, or (b) teach Tastypie to combine them. As I needed the latter, I wrote this:

from django.db.models.fields.related import ForeignKey, OneToOneField
from tastypie.resources import ModelResource
from .models import Item

class ItemResource(ModelResource):

    class Meta:
        queryset = Item.objects.all()
        resource_name = "items"

    def dehydrate(self, bundle):
        """
        Account for django-polymorphic
        """

        unacceptable_field_types = (ForeignKey, OneToOneField)

        for field in bundle.obj._meta.fields:
            if field.name in bundle.data:
                continue
            if not isinstance(field, unacceptable_field_types):
                bundle.data[field.name] = getattr(bundle.obj, field.name)

        return bundle.data

It's not the prettiest of solutions, but it seems to do the trick for me at this stage. If you're reading this and think I've missed something, please feel free to drop me a comment.