How to get the length of a cursor from mongodb using python?

Question

I'm looking for a feasible way to get the length of cursor got from MongoDB.

Heart

cursor.count()? – Shoifet

Deprecated in 3.7 – Montiel

Answer 1

It's actually as simple as

len(list(cursor))

Note that it will however consume the cursor.

Aseptic

as of Sept 2020, cursor.count() returns a '''DeprecationWarning: count is deprecated. Use Collection.count_documents instead.''' While count_document returns: '''AttributeError: 'Cursor' object has no attribute 'count_documents'''' So len(list(cursor)) is what works better. – Giza

When you say consume, do you mean move the position of the cursor, or does not convert the whole cursor contents (which could be huge) into a list? – Discomfiture

@Discomfiture I mean that the cursor is empty afterwards. I’d assume the content is converted into a list, which indeed may not be suitable for all usecases, but I didn’t test, maybe there are some optimizations indeed. – Aseptic

Answer 2

The cursor.count method is deprecated since pymongo 3.7.

The recommended method is to use the count_documents method of the collection.

Eremite

does this mean if you want the count before you iterate you have to query twice - once for the collection.count_documents() and another for collection.find()? – Stroh

Yes. One needs to call count_documents in an independent query. cursor.count also emitted an independent query, so this is not really a regression. However, this can be an issue for functions that take a cursor as input and need the cursor length. Those functions must be modified to also take the total as parameter, and caller code must be modified accordingly. (It might be possible to access collection and query filters from the cursor, as those are stored as double-underscored attributes, but that sounds bad.) – Albuquerque

@Jérôme, but how do you propose to get the collection from the cursor resulting from a find? – Bloodthirsty

@Bloodthirsty cursor.collection? – Albuquerque

Sorry, I didn't understand that I can apply the filter condition with count_documents(). Now it makes sense. – Bloodthirsty

*** AttributeError: 'Cursor' object has no attribute 'count' – Kakalina

Answer 3

The variant of the previous answer:

len(list(cursor.clone()))

doesn't consume cursor

Sanguinary

When you say consume, do you mean move the position of the cursor, or does not convert the whole cursor contents (which could be huge) into a list? – Discomfiture

Answer 4

cursor.count()

Counts the number of documents referenced by a cursor. Append the count() method to a find() query to return the number of matching documents. The operation does not perform the query but instead counts the results that would be returned by the query.

db.collection.find(<query>).count()

https://docs.mongodb.com/manual/reference/method/db.collection.count/

Amblyopia

cursor.count() is deprecated since pymongo 3.7. See my answer. – Albuquerque

cursor.count() is deprecated and the new countDocuments is more accurate but much slower see – Origan

Answer 5

len(list(cursor.clone())) worked really well for me, does not consume the editor so it can be use straight with your variable

Bialystok

Answer 6

According to the pymongo documentation, a Pymongo cursor, has a count method:

count(with_limit_and_skip=False)

By default this method returns the total length of the cursor, for example:

cursor.count()

If you call this method with with_limit_and_skip=True, the returned value takes limit and skip queries into account. For example, the following query will return 5 (assuming you have more than 5 documents):

cursor.limit(5).count(True)

Delvecchio

cursor.count() is deprecated since pymongo 3.7. See my answer. – Albuquerque

Answer 7

I find that using cursor.iter().count() is a feasible way to resolve this problem

Heart

Traceback (most recent call last): File "test.py", line 12, in <module> print (cursor.iter().count()) AttributeError: 'Cursor' object has no attribute 'iter' – Stroh

Answer 8

For some reason, some aggregations return an object that doesn't have the same methods, maybe different class, simple solution, convert the pseudo cursor to an array:

// A simple aggregation with `group`
var cursor = db.getCollection('collection').aggregate([
    {$match: {
        "property": {"$exists": true }
    }},
    {$group: { 
        _id: '$groupable',
        count: {$sum: 1}
    }},
    {$sort: {
        count: -1
    }}
]);

// Converting the "cursor" into an array
var cursor_array = cursor.toArray();

// Looping as an array using `for` instead of `while`
for (var i = 0; i < cursor_array.length; i++) {
    print(cursor_array[i]._id+'\t'+cursor_array[i].count);
}

Notice this solution is only for the shell, I don't know if this array method exists in other libraries.

Crayton

the OP asked for Python, you posted something closer to javascript – Railing

Answer 9

I was able to do count it this way:

def count():
    collection = db[col_name]
    count = collection.count_documents({"visited" : True})
    
    return count

Premiere

Answer 10

How about sum(1 for _ in cursor.clone()) so that you get the count but using constant memory instead of creating a new list. And you don't have to make another query to mongo with this solution.

Temperate

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Answer 11

Hello I was trying to find a solution to this and I figured it out.

#Count documents takes a filter object inside(a dict) and you need to call it
#after a Collection object not cursor.
portfolio.count_documents({})

I hope this helps

Sherrellsherrer