Get string array from Pymongo query

Question

I need to get an array with the values from the field 'colname'. I can't return a Cursor, just the array of values.

Is there a way to query this array without having to loop the Cursor? I feel this is a waste of processing resources.

Right now I'm doing this:

from pymongo import MongoClient

client = MongoClient('mongodb://localhost:27017/')
headers = client['headers']

entomo = headers.entomo

entomo_data = entomo.find()
entomo_array = []
for data in entomo_data:
    entomo_array.append(data['colname'])

Then I return the entomo_array.

Possible duplicate of PyMongo and toArray() method

Vince Bowdren
– Vince Bowdren

2017-03-28 21:16:58 +00:00
Commented Mar 28, 2017 at 21:16 — Vince Bowdren
– Vince Bowdren, Commented Mar 28, 2017 at 21:16
@VinceBowdren this is not a valid duplicate.

Sede
– Sede

2017-03-29 06:53:14 +00:00
Commented Mar 29, 2017 at 6:53 — Sede
– Sede, Commented Mar 29, 2017 at 6:53

Sede · Accepted Answer · 2017-03-29 07:01:23Z

1

You can do this with the .aggregate() method by $grouping your documents by None

cursor = entomo.aggregate([
    {'$group': {
        '_id': None, 
        'data': {'$push': '$colname'}
    }}
])

From there, you simply consume the cursor using next.

entomo_array = next(cursor)['data']

But if 'colname' is unique within the collection, you can simply use the the distinct method.

entomo_array = entomo.distinct('colname')

answered Mar 29, 2017 at 7:01

Sede

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Comments

Moi Syme · Accepted Answer · 2017-03-28 22:18:00Z

0

If the 'colname' field has distinct values or if you do not care about duplicate values you can use distinct function. For your example:

entomo_array = entomo.find().distinct('colname')

answered Mar 28, 2017 at 22:18

Moi Syme

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2 Comments

AFP_555 Over a year ago

I'm not sure but I think this would consume more resources than just looping the array. Wouldn't it? But I guess the distinct's implementation has a O(n) complexity as it must use a Hashmap, just like looping.

Moi Syme Over a year ago

About the execution time, I think that when you run distinct('colname') the command runs on mongodb server and then the result returns via one I/O but when you run a loop on cursor, generally I believe, the code will make more than one I/O. Finally, you can use the distinct command on collection level if you have not any filter. api.mongodb.com/python/current/api/pymongo/…

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