How to filter mongodb cursor result using python?

Question

I am writing a script where I need to fetch data from mongodb using python. I need to first sort data in descending order using timestamp field and take latest 100 documents and then again I need to sort those 100 documents based on using other fields (pin_code and timestamp). Here is the code:

cursor = db.col.find().sort([("timestamp", pymongo.DESCENDING)]).limit(100)
cus = cursor.sort([("pin_code", pymongo.ASCENDING),("timestamp",pymongo.DESCENDING)])

I am expecting that second cursor should return sorted data from first cusor but its NOT returning the expected result. I know mongodb find() and sort() function return cursor but can i use that cursor as an input to other cursor?

PS: I am using pymongo module

Can you explain what you are expecting and how this differs to what you are seeing. — Belly Buster
– Belly Buster, Commented Nov 3, 2019 at 23:14
Also please post example documents and what you have tried so far. — kevinadi
– kevinadi, Commented Nov 4, 2019 at 0:24

Belly Buster · Accepted Answer · 2019-11-04 21:56:26Z

The issue you have is that you can't compound multiple sorts (or, for that matter, limits) on the cursor object; only the last method call will have any effect. We can demonstrate this with this code:

from pymongo import MongoClient, DESCENDING, ASCENDING
import datetime

db = MongoClient()['mydatabase']

db.testcollection.delete_many({})
for i in range (0, 1000):
    db.testcollection.insert({'timestamp': datetime.datetime.now(), 'pin_code': i})

cursor = db.testcollection.find().sort([("timestamp", DESCENDING)]).limit(50)
cursor.limit(200)
cursor.limit(100)
cursor.limit(150)
print(len(list(cursor)))

Gives:

So what you need to do is use an aggregation pipeline, or something like this, with a single sort/filter and then use sort or sorted on the results:

from pymongo import MongoClient, DESCENDING
import datetime

db = MongoClient()['mydatabase']

db.testcollection.delete_many({})
for i in range (0, 1000):
    db.testcollection.insert({'timestamp': datetime.datetime.now(), 'pin_code': i})

my_list = list(db.testcollection.find().sort([("timestamp", DESCENDING)]).limit(100))
my_list = sorted(my_list, key = lambda x: x['timestamp'], reverse=True)
my_list = sorted(my_list, key = lambda x: x['pin_code'])
for item in my_list:
    print (item)

Amit Yadav · Accepted Answer · 2019-11-04 05:09:16Z

0

The find methods return a cursor, which is a reference to the result set of a query.

cursor.py

#!/usr/bin/python3

from pymongo import MongoClient

client = MongoClient('mongodb://localhost:27017/')

with client:

    db = client.testdb

    cars = db.cars.find()

    print(cars.next())
    print(cars.next())
    print(cars.next())

    cars.rewind()

    print(cars.next())
    print(cars.next())
    print(cars.next())    

    print(list(cars))

edited Nov 4, 2019 at 5:09

Amit Yadav

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answered Nov 4, 2019 at 3:04

Amit Tomar

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