LeMiz LeMiz - 3 days ago 5
Python Question

mongodb: insert if not exists

Every day, I receive a stock of documents (an update). What I want to do is insert each item that does not already exist.


  • I also want to keep track of the first time I inserted them, and the last time I saw them in an update.

  • I don't want to have duplicate documents.

  • I don't want to remove a document which has previously been saved, but is not in my update.

  • 95% (estimated) of the records are unmodified from day to day.



I am using the Python driver (pymongo).

What I currently do is (pseudo-code):

for each document in update:
existing_document = collection.find_one(document)
if not existing_document:
document['insertion_date'] = now
else:
document = existing_document
document['last_update_date'] = now
my_collection.save(document)


My problem is that it is very slow (40 mins for less than 100 000 records, and I have millions of them in the update).
I am pretty sure there is something builtin for doing this, but the document for update() is mmmhhh.... a bit terse.... (http://www.mongodb.org/display/DOCS/Updating )

Can someone advise how to do it faster?

Answer

Sounds like you want to do an "upsert". MongoDB has built-in support for this. Pass an extra parameter to your update() call: {upsert:true}. For example:

key = {'key':'value'}
data = {'key2':'value2', 'key3':'value3'};
coll.update(key, data, {upsert:true});

This replaces your if-find-else-update block entirely. It will insert if the key doesn't exist and will update if it does.

Before:

{"key":"value", "key2":"Ohai."}

After:

{"key":"value", "key2":"value2", "key3":"value3"}

You can also specify what data you want to write:

data = {"$set":{"key2":"value2"}}

Now your selected document will update the value of "key2" only and leave everything else untouched.

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