[python] How do I update a Mongo document after inserting it?

Let's say I insert the document.

post = { some dictionary }
mongo_id = mycollection.insert(post)

Now, let's say I want to add a field and update it. How do I do that? This doesn't seem to work.....

post = mycollection.find_one({"_id":mongo_id}) 
post['newfield'] = "abc"
mycollection.save(post)

mycollection.find_one_and_update({"_id": mongo_id}, 
                                 {"$set": {"newfield": "abc"}})

should work splendidly for you. If there is no document of id mongo_id, it will fail, unless you also use upsert=True. This returns the old document by default. To get the new one, pass return_document=ReturnDocument.AFTER. All parameters are described in the API.

The method was introduced for MongoDB 3.0. It was extended for 3.2, 3.4, and 3.6.

According to the latest documentation about PyMongo titled Insert a Document (insert is deprecated) and following defensive approach, you should insert and update as follows:

result = mycollection.insert_one(post)
post = mycollection.find_one({'_id': result.inserted_id})

if post is not None:
    post['newfield'] = "abc"
    mycollection.save(post)

This is an old question, but I stumbled onto this when looking for the answer so I wanted to give the update to the answer for reference.

The methods save and update are deprecated.

save(to_save, manipulate=True, check_keys=True, **kwargs)¶ Save a document in this collection.

DEPRECATED - Use insert_one() or replace_one() instead.

Changed in version 3.0: Removed the safe parameter. Pass w=0 for unacknowledged write operations.

update(spec, document, upsert=False, manipulate=False, multi=False, check_keys=True, **kwargs) Update a document(s) in this collection.

DEPRECATED - Use replace_one(), update_one(), or update_many() instead.

Changed in version 3.0: Removed the safe parameter. Pass w=0 for unacknowledged write operations.

in the OPs particular case, it's better to use replace_one.

I will use collection.save(the_changed_dict) this way. I've just tested this, and it still works for me. The following is quoted directly from pymongo doc.:

save(to_save[, manipulate=True[, safe=False[, **kwargs]]])

Save a document in this collection.

If to_save already has an "_id" then an update() (upsert) operation is performed and any existing document with that "_id" is overwritten. Otherwise an insert() operation is performed. In this case if manipulate is True an "_id" will be added to to_save and this method returns the "_id" of the saved document. If manipulate is False the "_id" will be added by the server but this method will return None.