如何在pymongo中获得数组长度？

Question

I watched this post How can I get the length of a MongoDb array field?

I tried these codes:

results=db.posts.find()
results[0].comments.length

I am sure the post and comments exist. But it does not work. It will return an error:

AttributeError: 'comments' object has no attribute 'length'

How to fix it?

Answer 1

Pymongo returns documents as dicts. find() returns an array of dicts. You need:

len(results[0]['comments'])

Answer 2

If you just want the length of the array, do:

result = client.aggregate([
    {
        '$project': {
            'my_array_length': {'$size': '$my_array'}
        }
    }
])