2.8. Signals

Added in version 0.5.

2.8.1. Overview

Signals are found within the mongoengine.signals module. Unless specified, signals receive no additional arguments beyond the sender class and document instance. Post-signals are only called if there were no exceptions raised during the processing of their related function.

Available signals include:

pre_init

Called during the creation of a new Document or EmbeddedDocument instance, after the constructor arguments have been collected but before any additional processing has been done to them. (I.e. assignment of default values.) Handlers for this signal are passed the dictionary of arguments using the values keyword argument and may modify this dictionary prior to returning.

post_init

Called after all processing of a new Document or EmbeddedDocument instance has been completed.

pre_save

Called within save() prior to performing any actions.

pre_save_post_validation

Called within save() after validation has taken place but before saving.

post_save

Called within save() after most actions (validation, insert/update, and cascades, but not clearing dirty flags) have completed successfully. Passed the additional boolean keyword argument created to indicate if the save was an insert or an update.

pre_delete

Called within delete() prior to attempting the delete operation.

post_delete

Called within delete() upon successful deletion of the record.

pre_bulk_insert

Called after validation of the documents to insert, but prior to any data being written. In this case, the document argument is replaced by a documents argument representing the list of documents being inserted.

post_bulk_insert

Called after a successful bulk insert operation. As per pre_bulk_insert, the document argument is omitted and replaced with a documents argument. An additional boolean argument, loaded, identifies the contents of documents as either Document instances when True or simply a list of primary key values for the inserted records if False.

2.8.2. Attaching Events

After writing a handler function like the following:

import logging
from datetime import datetime

from mongoengine import *
from mongoengine import signals

def update_modified(sender, document):
    document.modified = datetime.utcnow()

You attach the event handler to your Document or EmbeddedDocument subclass:

class Record(Document):
    modified = DateTimeField()

signals.pre_save.connect(update_modified)

While this is not the most elaborate document model, it does demonstrate the concepts involved. As a more complete demonstration you can also define your handlers within your subclass:

class Author(Document):
    name = StringField()

    @classmethod
    def pre_save(cls, sender, document, **kwargs):
        logging.debug("Pre Save: %s" % document.name)

    @classmethod
    def post_save(cls, sender, document, **kwargs):
        logging.debug("Post Save: %s" % document.name)
        if 'created' in kwargs:
            if kwargs['created']:
                logging.debug("Created")
            else:
                logging.debug("Updated")

signals.pre_save.connect(Author.pre_save, sender=Author)
signals.post_save.connect(Author.post_save, sender=Author)

Finally, you can also use this small decorator to quickly create a number of signals and attach them to your Document or EmbeddedDocument subclasses as class decorators:

def handler(event):
    """Signal decorator to allow use of callback functions as class decorators."""
    def decorator(fn):
        def apply(cls):
            event.connect(fn, sender=cls)
            return cls
        fn.apply = apply
        return fn
    return decorator

Using the first example of updating a modification time the code is now much cleaner looking while still allowing manual execution of the callback:

@handler(signals.pre_save)
def update_modified(sender, document):
    document.modified = datetime.utcnow()

@update_modified.apply
class Record(Document):
    modified = DateTimeField()