Testing MongoDB aggregation problem. Creates 10 posters and randomly assigns them to 100 posts. Aggregation query returns mapping of poster to latest post sorted by creation date in descending order.

aggregation_test.py

from pymongo import MongoClient
import datetime
import bunch
from faker import Faker
from random import choice

client = MongoClient(host="localhost", port=27017)

faker   = Faker()
posters = []
for _ in range(0, 10):
    posters.append(faker.name())

for _ in range(0, 100):
    post = {
          'poster': choice(posters)
        , 'text': faker.lorem()
        , 'created_at': datetime.datetime.utcnow()
    }

    client.aggr.posts.insert(post)

if __name__ == '__main__':
    pipelines = []
    pipelines.append({ '$sort': { 'created_at': -1 } })
    pipelines.append({ '$project': { 'poster': '$poster' } })
    pipelines.append({ '$group': { '_id': '$poster', 'post_id': { '$first': '$_id' } } })
    pipelines.append({ '$skip': 0 })
    pipelines.append({ '$limit': 10 })

    results = client.aggr.posts.aggregate(pipelines)

    print results['result']