final7-solution.py

# coding: utf8

import pymongo


connection = pymongo.MongoClient('localhost', 27017)
db = connection.photos
albums = db.albums
images = db.images


def remove_orphan():
    cursor = albums.find()
    cursor2 = images.find()
    images_list = []

    for item in cursor:
        images_ids = item["images"]
        for id in images_ids:
            if not id in images_list:
                images_list.append(id)

    c = 0
    for image in cursor2:
        id = image["_id"]
        if not id in images_list:
            #print id
            c += 1
            images.remove({"_id": id})
    print c

remove_orphan()