mongoengine slow serialization of embedded documents with reference fields - pymongo

Question

I have a small DB with about 500 records. I'm trying to implement a versioning scheme where I save the form along with its current version to my Record collection. Ideally, I would like to store the form along with its version number in an embedded document to keep things nice and tidy:
class Structure(db.EmbeddedDocument):
form = db.ReferenceField(Form, required = True)
version = db.IntField(required = True)
#property
def short(self):
return {
'form': self.form,
'version': self.version
}
class Record(db.Document):
structure = db.EmbeddedDocumentField(Structure)
#property
def short(self):
return {
'structure': self.structure.short
}
This way when I recall a record I can grab the form and the version that was used at the time. Running some timing tests:
start = time.clock()
records = Record.objects.select_related()
print ('Time: ', time.clock() - start)
response = [i.short for i in records]
print ('Time: ', time.clock() - start)
I find the query time for all records Record.objects.select_related() to be reasonable at, ~ 1.12 s, however, I'm finding serialization for the purpose of JSON transfer is extremely expensive at ~ 24.1s!
If I make a slight modification by removing use of the EmbeddedDocument:
class Record(db.Document):
form = db.ReferenceField(Form, required = True)
version = db.IntField(required = True)
#property
def short(self):
return {
'form': self.form,
'version': self.version
}
Running the same test I find the query time to be pretty much unchanged at ~ 1.36 s, however, the serialization time improved by 24x to 1.14s. I really do not understand why use of an embedded document would lead to such as massive penalty in serialization time...? Is dereferencing in an embedded object more difficult?