Calling certain functions when initializing a new mongoengine document


I’m trying to define a Document class that has some fields that have their values automatically calculated when a new document is created.
So far I have succeeded in doing that by extending the __init__ function like this:

class URLDoc(Document):
    url = fields.URLField(requierd=True)
    domain = fields.StringField(required=True)
    screenshot = fields.ImageField(required=True, size=(600, 600, True), thumbnail=(100, 100, True))

    def __init__(self, *args, **kwargs):
        super(URLDoc, self).__init__(*args, **kwargs)
        self.domain = urlparse(self.url).netloc

    def get_and_save_screenshot(self):
        '''Some code to get a screenshot of the website on self.url'''


This way I was able to create new documents in mongo by just calling new_urldoc = URLDoc(url="some url") and then just

When I started loading existing documents from mongo by URLDoc.objects.get(id="some id") I realized that the __init__ function will be triggered again set new data (take a new screenshot for example) in the document.

I want to implement this but only when the document is new, I looked up everywhere and couldn’t find an answer..

Is there a way to call certain functions when initializing a new document as opposed to initializing an existing document?

Asked By: guysh



I got it working, I don’t know if it’s a work around or not but I check for self._id (an id is created only on save) on the init function and if there’s none (meaning it’s a new document), I set the values.

Answered By: guysh

I was looking for a similar solution and after a while I found that MongoEngine supports Signals. There are multiple signals that can be coupled with document before/after certain operations:

  • initialization
  • saving
  • deletion

Unfortunately is does not support signals based on document update, but a workaround it is to check if the sender document has an id or not. A simple snippet code – based on the example in the MongoEngine documentation:

def update_modified(sender, document):
        document.modified = datetime.utcnow() # Will only be executed if the document has been already saved
Answered By: Mosallamy
Categories: questions Tags: ,
Answers are sorted by their score. The answer accepted by the question owner as the best is marked with
at the top-right corner.