# 2.8. Signals

Added in version 0.5.

Note

Signal support is provided by the excellent blinker library. If you wish to enable signal support this library must be installed, though it is not required for MongoEngine to function.

## 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)

Warning

Note that EmbeddedDocument only supports pre/post_init signals. pre/post_save, etc should be attached to Document’s class only. Attaching pre_save to an EmbeddedDocument is ignored silently.

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()