Override save, put, get, etc… methods in Google App Engine

Question

Is it possible to override methids for db.Model in Google App Engine? I want to declare beforeSave, afterSave methods, etc.. to create automatic tagging system.

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Answer 1

I've attempted to improve upon the answer by @jbochi:

1. According to https://developers.google.com/appengine/docs/python/datastore/modelclass#Model_put , put() should return a Key, so the return value from the library should be passed through.
2. db.Model.save(), while deprecated, is (a) sometimes still used, (b) meant to be a synonym for put() and (c) apparently not called directly by put() - so should be handled manually.

Revised code:

class HookedModel(db.Model):
  def before_put(self):
    pass

  def after_put(self):
    pass

  def put(self, **kwargs):
    self.before_put()
    result = super(HookedModel, self).put(**kwargs)
    self.after_put()
    return result

  def save(self, **kwargs):
    self.before_put()
    result = super(HookedModel, self).save(**kwargs)
    self.after_put()
    return result

You should still read http://blog.notdot.net/2010/04/Pre--and-post--put-hooks-for-Datastore-models if you wish to use the monkeypatching, or Chris Farmiloe's answer for use of the async methods.

Answer 2

Yes, it's possible to override these methods. Have a look at this blog post by Nick Johnson.The hooked model class looks this:

class HookedModel(db.Model):
  def before_put(self):
    pass

  def after_put(self):
    pass

  def put(self, **kwargs):
    self.before_put()
    super(HookedModel, self).put(**kwargs)
    self.after_put()

Read the blog to see how to handle the db.put() method too.

You might also be interested on "derived properties".

Answer 3

I posted an extension to jbochi's HookedModel class so that the before_put and after_put methods are correctly invoked when called from db.put() and the _async family of functions.

See AppEngine PreCall API hooks