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MongoEngine - Signals

Signals are events dispatched by a sender object, any number of receiver objects can subscribe to such events. A signal receiver can subscribe to a specific sender or may receive signals from many senders.

In MongoEngine, signal handpng is supported by bpnker pbrary, which means you need to install it using pip utipty. The mongoengine.signals module has the definitions of following signals −

pre_init	Called during the creation of a new Document or EmbeddedDocument instance and executed after the constructor arguments have been collected but before any additional processing has been done to them.
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_vapdation	Called within save() after vapdation has taken place but before saving.
post_save	Called within save() after most actions (vapdation, insert/update) have completed successfully. An additional Boolean keyword argument is passed 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 vapdation of the documents to insert, but prior to any data being written.
post_bulk_insert	Called after a successful bulk insert operation. An additional Boolean argument, loaded, identifies the contents of documents as either Document instances when True or a pst of primary key values for the inserted records if False.

An event handler function is then attached to Document class. Note that EmbeddedDocument only supports pre/post_init signals. pre/post_save, etc., should be attached to Document’s class only.

You can also use a decorator to quickly create a number of signals and attach them to your Document or EmbeddedDocument subclasses as class decorators.

In the following example, used as demonstration of signal handlers, we also use Python’s standard pbrary module – logging and set the logging level to debug.


from mongoengine import *
from mongoengine import signals
import logging
logging.basicConfig(level=logging.DEBUG)

We then write a document class so that corresponding collection is created in newdb database. Inside the class, two class mehods pre_save() and post_save() methods are defined which are intended to be invoked before and after a document is saved in Author collection.


class Author(Document):
   name = StringField()
   
   def __unicode__(self):
      return self.name

   @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")

Both the class methods are defined with arguments for classname, sender object and document with optional pst of keyword arguments.

Finally, we register the signal handlers.

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

As we create an instance of Document subclass, the console log will show the pre and post save signals being processed by respective event handlers.


Author(name="Lathkar").save()

Python console reports the log as shown below −


DEBUG:root:Pre Save: Lathkar
DEBUG:root:Post Save: Lathkar
DEBUG:root:Created

MongoEngine - Signals

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