As discussed at https://github.com/mongodb/django-mongodb-backend/pull/407, loading a model with an EmbeddedModelField or PolymorphicEmbeddedModelField where on of the embedded model field's isn't present in a document crashes. 

Data could be missing if the user is using an existing db with sparse data, or if they added a field to an existing embedded model, since schema changes on embedded models aren't yet supported.