Handle nested OneToOneField when related object doesn't exist

[dsanders11]

This PR fixes the case where a nested field is a OneToOneField and the related object doesn't exist. Attempting to access the attribute throws a RelatedObjectDoesNotExist exception which causes django-elasticsearch-dsl to not be able to index the model.

Fix simply checks if the exception thrown is an ObjectDoesNotExist exception (which is a subclass of AttributeError) and if so returns None for the instance, allowing the field to be skipped when the related object doesn't exist.

Example reproduce code:

# models.py

class Car(models.Model):
    name = models.CharField()
    color = models.CharField()
    manufacturer = models.ForeignKey('Manufacturer')

class Manufacturer(models.Model):
    name = models.CharField()
    country_code = models.CharField(max_length=2)
    created = models.DateField()
    headquarters = models.OneToOneField('Headquarters')

class Headquarters(models.Model):
    city = models.CharField()

# documents.py

from django_elasticsearch_dsl import DocType, Index
from .models import Car

car = Index('cars')
car.settings(
    number_of_shards=1,
    number_of_replicas=0
)


@car.doc_type
class CarDocument(DocType):
    manufacturer = fields.ObjectField(properties={
        'name': fields.StringField(),
        'country_code': fields.StringField(),
        'headquarters': fields.ObjectField(properties={
            'city': fields.StringField()
        })
    })

    class Meta:
        model = Car
        fields = [
            'name',
            'color',
        ]

[stevelacey]

You could catch the ObjectDoesNotExist before the except block that catches AttributeError, instead of doing an isinstance check

[dsanders11]

@stevelacey, you're right, that's much cleaner, not sure what I was thinking. I'll update the PR to use that style.

[sabricot]

Thanks @dsanders11 !