MongoDb query condition on comparing 2 fields

Question

I have a collection T, with 2 fields: Grade1 and Grade2, and I want to select those with condition Grade1 > Grade2, ho

Answer 1

In case performance is more important than readability and as long as your condition consists of simple arithmetic operations, you can use aggregation pipeline. First, use $project to calculate the left hand side of the condition (take all fields to left hand side). Then use $match to compare with a constant and filter. This way you avoid javascript execution. Below is my test in python:

import pymongo
from random import randrange

docs = [{'Grade1': randrange(10), 'Grade2': randrange(10)} for __ in range(100000)]

coll = pymongo.MongoClient().test_db.grades
coll.insert_many(docs)

Using aggregate:

%timeit -n1 -r1 list(coll.aggregate([
    {
        '$project': {
            'diff': {'$subtract': ['$Grade1', '$Grade2']},
            'Grade1': 1,
            'Grade2': 1
        }
    },
    {
        '$match': {'diff': {'$gt': 0}}
    }
]))

1 loop, best of 1: 192 ms per loop

Using find and $where:

%timeit -n1 -r1 list(coll.find({'$where': 'this.Grade1 > this.Grade2'}))

1 loop, best of 1: 4.54 s per loop