Simple script not printing

Question

I'm trying to sort a collection, and then print the first 5 docs to make sure it has worked:

#!/user/bin/env python



import pymongo



# Establish a connection to the mongo database.

connection = pymongo.MongoClient('mongodb://localhost')



# Get a handle to the students database.

db = connection.school

students = db.students





def order_homework():



    projection = {'scores': {'$elemMatch': {'type': 'homework'}}}

    cursor = students.find({}, projection)



    # Sort each item's scores.

    for each in cursor:

        each['scores'].sort()



    # Sort by _id.

    cursor = sorted(cursor, key=lambda x: x['_id'])



    # Print the first five items.

    count = 0

    for each in cursor:

        print(each)

        count += 1

        if count == 5:

            break





if __name__ == '__main__':

    order_homework()

When I run this, nothing prints.

If I take out the sorts, then it prints.

Each sort works when run individually.

Please teach me what I'm doing wrong / educate me.
Thank you.

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Answer 1

You're trying to treat the cursor like a list, which you can iterate several times from the start. PyMongo cursors don't act that way - once you've iterated it in for each in cursor, the cursor is completed and you can't iterate it again.

You can turn the cursor into a list like:

data = list(students.find({}, projection))

For efficiency, get results pre-sorted from MongoDB:

list(students.find({}, projection).sort('_id'))

This sends the sort criterion to the server, which then streams the results back to you pre-sorted, instead of requiring you to do it client-side. Now delete your "Sort by _id" line below.

Simple script not printing

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