Updating a value of dictionary in list of dictionaries in a collection using PyMongo

Question

The collection structure that I have is as follows:

defaultdict(
    lambda: {
        '_id' : None,
        'stuff' : [defaultdict(lambda : 0)]
    })

I am trying to initialise a list of dictionaries that I'll keep on updating in a way that if a dictionary's key already exists then I'll increase its value by 1 otherwise I'll update the list of dictionary with the new key, value pair. e.g. is the value of stuff is [] and I have come across a value val then the value of stuff will be [{'val' : 1}] and further if I get a value val2 then stuff = [{'val' : 1}, {'val2' : 1}] and again if I get val, stuff should be [{'val' : 2}, {'val2' : 1}].

I tried this:

table.update({'_id' : data['_id']}, \
             {'$inc' : {"stuff.$."+keyvalue : 1}})

where, data is a JSON object having an _id and a list of dictionaries stuff. Running this I received an OperationFailure: The positional operator did not find the match needed from the query.

I cannot figure out what to do? I am a Mongo newbie.

What do you mean by *if the dictionary already exists then I'll update its value by 1* ? what is `keyvalue`? can you show use your document? — styvane, Mar 22 '15 at 08:24
@Michael I have updated the question with a better explanation. I hope it gives a better idea of what I asking? — Animesh Pandey, Mar 22 '15 at 08:57

styvane · Accepted Answer · 2015-03-22T12:54:40.547

Quoting the documentation

When used with update operations, e.g. db.collection.update() and db.collection.findAndModify(),

the positional $ operator acts as a placeholder for the first element that matches the query document, and

the array field must appear as part of the query document.

So the stuff array should appear in your query. Now since you are doing conditional update you need to check if your array already has any subdocument with your keyvalue using $exists.

if not table.find_one({ "_id": data['_id'], "stuff": {"$elemMatch": {keyvalue: { "$exists" : True }}}}):
    table.update({ "_id": data['_id'] }, { "$push": { "stuff": { keyvalue: 1 }}})
else:
    table.update({ "_id": data['_id'], "stuff": { "$elemMatch": { keyvalue: { "$exists" : True}}}}, { "$inc": { "stuff.$." + keyvalue: 1}})

In the first part should I also add `{upsert : True}`? – Animesh Pandey Mar 22 '15 at 20:40 — Animesh Pandey, Mar 22 '15 at 20:40
why will want that? you do not need to – styvane Mar 22 '15 at 20:43 — styvane, Mar 22 '15 at 20:43

Updating a value of dictionary in list of dictionaries in a collection using PyMongo

1 Answers1