Python - How to select ObjectId

Question

Python noob here. I have the following in my database in an object called "data" and I want to write some python that will select the lowest score from each group of student_id....and grab the unique ObjectId for each minimum record found?

{u'student_id': 197, u'_id': ObjectId('50906d7fa3c412bb040eb88d'), u'type': u'homework', u'score': 10.90872422518918}
{u'student_id': 197, u'_id': ObjectId('50906d7fa3c412bb040eb88e'), u'type': u'homework', u'score': 88.3871242475841}
{u'student_id': 198, u'_id': ObjectId('50906d7fa3c412bb040eb892'), u'type': u'homework', u'score': 17.46279901047208}
{u'student_id': 198, u'_id': ObjectId('50906d7fa3c412bb040eb891'), u'type': u'homework', u'score': 76.18366499496366}
{u'student_id': 199, u'_id': ObjectId('50906d7fa3c412bb040eb895'), u'type': u'homework', u'score': 49.34223066136407}
{u'student_id': 199, u'_id': ObjectId('50906d7fa3c412bb040eb896'), u'type': u'homework', u'score': 58.09608083191365}

Thank you for your trouble....here is my code...

import pymongo

from itertools import groupby
from pymongo import Connection


connection = Connection('localhost', 27017)

db = connection.students

data = db.grades.find({'type' : 'homework'}).sort([('student_id',pymongo.ASCENDING),('score',pymongo.ASCENDING)])



for student_id, items in groupby(data, lambda s: s['student_id']):
    lowest_score = min(i['score'] for i in items)

    print lowest_score

I can print out the lowest score per student but now I need to grab the unique ObjectId for each lowest_score. — thefonso
– thefonso, Commented Nov 9, 2012 at 23:46

Blckknght · Accepted Answer · 2012-11-09 23:56:26Z

2

I think you may be able to do your minimum check differently and get the ObjectID rather than the score from the minimum value. Here's how I'd do it:

lowest_id = min(items, key=lambda i: i['score'])['_id']

Now, it's possible (indeed, likely) that you can do something similar in the database itself, and probably that will have better performance. However, the above may work just fine if your data set is not too unwieldy.

answered Nov 9, 2012 at 23:56

Blckknght

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2 Comments

redShadow Over a year ago

yep, I was about to answer that.. this is definitely the right way

thefonso Over a year ago

yep...this one seems to be it.

Cameron Sparr · Accepted Answer · 2012-11-09 23:54:26Z

1

not sure if you still have access to the student_id field, if so you could replace:

lowest_score = min(i['score'] for i in items)

with:

lowest_score = min((i['score'],i['student_id']) for i in items)

answered Nov 9, 2012 at 23:54

Cameron Sparr

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4 Comments

Cameron Sparr Over a year ago

(min will compare the first element in each tuple)

thefonso Over a year ago

ah...I just stumbled on this also...so then how would I say ...print or isolate the student_id at that point? (thank you again for your time)

Cameron Sparr Over a year ago

student_id = lowest_score[1]

Cameron Sparr Over a year ago

lowest_score = lowest_score[0], I probably should have renamed the tuple in the statement above

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