Signals pre_save is triggered on the parent document not the embedded one

[charli-polo]

Hi there,

I tried to auto update a timestamp on a EmbeddedDocument, but when calling save() on the EmbeddedDocument document, the signals receive an instance of the parent document...

Is this a bug or I am doing something wrong ?

class Item(db.EmbeddedDocument):
    url = db.StringField(primary_key=True)
    title = db.StringField()
    modified = db.DateTimeField()

class Search(db.Document):
    name = db.StringField(required=True)
    items = db.EmbeddedDocumentListField(Item)
 
def update_modified(sender, document):
    logging.warning(sender)
    logging.warning(document)

signals.pre_save.connect(update_modified)

[bagerard]

Hi, When you .save() an EmbeddedDocument, it is actually simply forwarding the call to the parent Document's .save() https://github.com/MongoEngine/mongoengine/blob/master/mongoengine/document.py#L91

I understand the confusion though, since there is a .save() method on EmbeddedDocument, you may expect to have a pre_save signal to be fired but the only class that triggers the pre_save signal is the Document

mongoengine/mongoengine/document.py

Line 359 in d074194

signals.pre_save.send(self.__class__, document=self, **signal_kwargs)

It's not a bug, being able to call .save() from the EmbeddedDocument is handy in some circumstances but I think it should generally be avoided (and call .save() from the parent Document instead) because it's misleading.

I would also recommend to use the 'sender' argument when setting up the signals:

signals.pre_save.connect(update_modified, sender=Search)

I believe the documentation could be improved to make this clear. I'll send a PR shortly