Append item to MongoDB document array in PyMongo without re-insertion

Question

I am using MongoDB as the back-end database for Python web application (PyMongo + Bottle). Users can upload files and optionally 'tag' these files during upload. The tags are stored as a list within the document, per below:

{
    "_id" : ObjectId("561c199e038e42b10956e3fc"),
    "tags" : [ "tag1", "tag2", "tag3" ],
    "ref" : "4780"
}

I am trying to allow users to append new tags to any document. I came up with something like this:

def update_tags(ref, new_tag)
    # fetch desired document by ref key as dict
    document = dict(coll.find_one({'ref': ref}))
    # append new tag
    document['tags'].append(new_tag)
    # re-insert the document back into mongo
    coll.update(document)

(fyi; ref key is always unique. this could easily be _id as well.) It seems like there should be a way to just update the 'tags' value directly without pulling back the entire document and re-inserting. Am I missing something here?

Any thoughts are greatly appreciated :)

Answer 1

You don't need to use to retrieve the document first just use the .update method with the $push operator.

def update_tags(ref, new_tag):
    coll.update({'ref': ref}, {'$push': {'tags': new_tag}})

Since update is deprecated you should use the find_one_and_update or the update_one method if you are using pymongo 2.9 or newer