Never writing default values in DB if key not present

Default values are never written to DB if containing prop was removed before:

import mongoengine

class Embdoc(mongoengine.EmbeddedDocument):
    prop = mongoengine.IntField(default=0)

class Doc(mongoengine.Document):
    emb = mongoengine.EmbeddedDocumentField(Embdoc, default=Embdoc)

    meta = {
        "db_alias": "db_name",
        "collection": "collection"
    }

mongoengine.connection.register_connection('db_name',name='db_name',host='localhost')
collection = mongoengine.connection.get_db('db_name').collection

collection.remove()
o = Doc()
o.save()
print(o.emb.prop)                             # 0
print(collection.find().limit(1)[0])          # {u'_id': ObjectId('5abaaaaaaaaaaaaaaaaaaaaa'), u'emb': {u'prop': 0}}
collection.update({}, {"$unset": {"emb":1}})
o.reload()
o.save()
print(o.emb.prop)                             # 0
print(collection.find().limit(1)[0])          # {u'_id': ObjectId('5abaaaaaaaaaaaaaaaaaaaaa')}

# Even so
o.emb.prop = 0
o.save()
print(o.emb.prop)                             # 0
print(collection.find().limit(1)[0])          # {u'_id': ObjectId('5abaaaaaaaaaaaaaaaaaaaaa')}

Funny isn't it ?

python -c "import mongoengine; print(mongoengine.__version__)" -> 0.15.0
ubuntu 17.10 artful
kernel 4.13.0

Start by running the Python reproducer against MongoDB and compare the document before and after the external $unset and save calls. Trace the save and reload entry points involved in EmbeddedDocumentField and default values. Done means the reproduced missing embedded default is persisted, with regression coverage for the shown sequence.