How to find the maximum value of a document field with Python and MongoDB?

Question

I am trying to find a document in a MongoDB instance with the largest value of a particular field.

Here is the current code I have to do that:

document = collection.find().sort('experiment_id', -1).limit(1)

if document is None:
    # ...
else:
    return document['experiment_id']

I am using this query format (find() followed by sort() combined with limit(1) because this is supposed to be interpreted by Mongo in a highly optimized way.

However, this does not work because document is a cursor type, therefore one cannot call ['experiment_id'] on it.

What I find slightly suprising is that limit(1) returns a cursor object and not a single value or None type.

How should I write a pymongo query which finds the largest value of the field 'experiment_id' from a collection of documents?

It seems obvious that the following will work but it doesn't seem like a particularly good solution.

for document in documents:
    return document['experiment_id']

Answer 1

Since you want only one result - as a document, rather than a cursor, you could use find_one() with a sort criteria which behaves like it does as a param to find(), and drop the limit clause, since it will be ignored anyway.

find_one(filter=None, *args, **kwargs)

Returns a single document, or None if no matching document is found.

document = collection.find_one({}, sort={'experiment_id': -1})

if document is None:
    # ...
else:
    return document['experiment_id']

Additionally, if you want just the 'experiment_id' and none of the rest, you can use a projection which would return a document with only the specified fields.

document = collection.find_one(
    {},
    sort={'experiment_id': -1},
    projection={'_id': False, 'experiment_id': True}  # only experiment_id
)

if document:
    return document['experiment_id']
# else logic here