How to autoincrement id at every insert in Pymongo

Question

I am using pymongo to insert documents in the mongodb. here is code for router.py file

    temp = db.admin_collection.find().sort( [("_id", -1)] ).limit(1)
    for doc in temp:
        admin_id = str(int(doc['_id']) + 1)


    admin_doc ={
    '_id'       :   admin_id,
    'question'  :   ques,
    'answer'    :   ans,
    }
    collection.insert(admin_doc)

what should i do so that at every insert _id is incremented by 1.

Answer 1

If you want to manually change the "_id" value you can do this by changing the _id value in the returned document. You could do this in a similar manner to the way you have proposed in your question. However I do not think this approach is advisable.

curs = db.admin_collection.find().sort( [("_id", -1)] ).limit(1)
for document in curs:
    document['_id'] = str(int(document['_id']) + 1)
    collection.insert(document)

It is generally not a good idea to manually make your own id's. These values have to be unique and there is no guarantee that str(int(document['_id']) + 1) will always be unique.

Instead if you want to duplicate the document you can delete the '_id' key and insert the document.

curs = db.admin_collection.find().sort( [("_id", -1)] ).limit(1)
for document in curs:
    document.pop('_id',None)
    collection.insert(document)

This inserts the document and allows mongo to generate the unique id.

Answer 2

It doesn’t seem like a very good idea, but if you really want to go through with it you can try setup like below.

It should work good enough in a low traffic application with single server, but I wouldn't try anything like this with replicated or sharded enviroment or if you perform large amount of inserts.

Create separate collection to handle id seqs:

db.seqs.insert({
    'collection' : 'admin_collection',
    'id' : 0
})

Whenever you need to insert new document use something similar to this:

def insert_doc(doc):
    doc['_id'] = str(db.seqs.find_and_modify(
        query={ 'collection' : 'admin_collection' },
        update={'$inc': {'id': 1}},
        fields={'id': 1, '_id': 0},
        new=True 
    ).get('id'))

    try:
        db.admin_collection.insert(doc)

    except pymongo.errors.DuplicateKeyError as e:
        insert_doc(doc)