Why get_value_from_instance in ObjectField returns {} instead of None if related object doesn't exists?

[HardGITer]

Example structure:
Models:

class Author(models.Model):
    name = models.CharField(max_length=30) 

class Article(models.Model):
    name = models.CharField(max_length=100)
    author = models.ForeignKey(Author, on_delete=models.CASCADE)

Documents:

@registries.registry.register_document
class AuthorDocument(Document):
    class Index:
        name = 'author'

    class Django:
        model = models.Author
        fields = ['name']

@registries.registry.register_document
class ArticleDocument(Document):
    author = fields.ObjectField(doc_class=AuthorDocument)

    class Index:
        name = 'article'

    class Django:
        model = models.Article
        fields = ['name']

Create and index article without tags:

art = Article(name="article 1")
art.tag_set  # returns None
# call index logic

For such records we'll get structure to serialize in elasticsearch:

{
    "name": "article 1",
    "tag": {},
}

The main question is why do you return {} instead of None in get_value_from_instance and because of this the field is indexed as {} instead of None?
Django orm returns None in such cases. Maybe better to have similar behavior with Django here? Because difference in concepts breaks some serializer logic: when i use another serializer as serializer field, it tries to serialize related object(when it doesn't exists but elasticsearch returns {}) and i get error because {} doesn't have any fields

[oehrlein]

@HardGITer Can you see if the changes I made here fix your problem: #302

If I'm understanding your issue correctly, then we have/had similar problems.