Mongo Db array two Key Compare [duplicate] - mongodb

Question

I have a collection T, with 2 fields: Grade1 and Grade2, and I want to select those with condition Grade1 > Grade2, how can I get a query like in MySQL?
Select * from T Where Grade1 > Grade2

Answer 1

You can use a $where. Just be aware it will be fairly slow (has to execute Javascript code on every record) so combine with indexed queries if you can.
db.T.find( { $where: function() { return this.Grade1 > this.Grade2 } } );
or more compact:
db.T.find( { $where : "this.Grade1 > this.Grade2" } );
UPD for mongodb v.3.6+
you can use $expr as described in recent answer

Answer 2

You can use $expr ( 3.6 mongo version operator ) to use aggregation functions in regular query.
Compare query operators vs aggregation comparison operators.
Regular Query:
db.T.find({$expr:{$gt:["$Grade1", "$Grade2"]}})
Aggregation Query:
db.T.aggregate({$match:{$expr:{$gt:["$Grade1", "$Grade2"]}}})

Answer 3

If your query consists only of the $where operator, you can pass in just the JavaScript expression:
db.T.find("this.Grade1 > this.Grade2");
For greater performance, run an aggregate operation that has a $redact pipeline to filter the documents which satisfy the given condition.
The $redact pipeline incorporates the functionality of $project and $match to implement field level redaction where it will return all documents matching the condition using $$KEEP and removes from the pipeline results those that don't match using the $$PRUNE variable.
Running the following aggregate operation filter the documents more efficiently than using $where for large collections as this uses a single pipeline and native MongoDB operators, rather than JavaScript evaluations with $where, which can slow down the query:
db.T.aggregate([
{
"$redact": {
"$cond": [
{ "$gt": [ "$Grade1", "$Grade2" ] },
"$$KEEP",
"$$PRUNE"
]
}
}
])
which is a more simplified version of incorporating the two pipelines $project and $match:
db.T.aggregate([
{
"$project": {
"isGrade1Greater": { "$cmp": [ "$Grade1", "$Grade2" ] },
"Grade1": 1,
"Grade2": 1,
"OtherFields": 1,
...
}
},
{ "$match": { "isGrade1Greater": 1 } }
])
With MongoDB 3.4 and newer:
db.T.aggregate([
{
"$addFields": {
"isGrade1Greater": { "$cmp": [ "$Grade1", "$Grade2" ] }
}
},
{ "$match": { "isGrade1Greater": 1 } }
])

Answer 4

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