MongoDB: update all document on one field - mongodb

Question

{
"_id" : 1,
"users" : 2329255
},
{
"_id" :2,
"users" : 2638831
}
how to update all documents users field divided by 100.
result will be
{
"_id" : 1,
"users" : 23292.55
},
{
"_id" : 2,
"users" : 26388.31
}
db.coll.update({}, {$set: {'users': {'$divide': ['$users', 100]}}})
----its not working

Answer 1

Try below query:
db.coll.find().snapshot().forEach(
function (e) {
e.users = e.users/100;
// save the updated document
db.coll.save(e);
}
)
Above query will change/update the data in DB. If you want to fetch records with devided value then use $ project:
db.coll.aggregate(
[
{ $project: { users: { $divide: [ "$users", 100 ] } } }
]
)
this will not update the data but will return you desired value.
Use as per your requirement.

Answer 2

The $divide operator is only valid for the aggregate() function, not the update() function. What you want to do is use the aggregate() method to create a computed field, iterate the results from
the aggregate() cursor to create bulk update operations that you can send to the server in one request, rather that sending each update request with each item in the result.
The following example demonstrates this:
var bulkUpdateOps = [];
db.coll.aggregate([
{ "$match": { "users": { "$exists": true } } }
{
"$project": {
"computed_field": {
"$divide": ["$users", 100]
}
}
}
]).forEach(function(doc){
bulkUpdateOps.push({
"updateOne": {
"filter": { "_id": doc._id },
"update": { "$set": { "users": doc.computed_field } }
}
});
if (bulkUpdateOps.length === 500) {
db.coll.bulkWrite(bulkUpdateOps);
bulkUpdateOps = [];
}
});
if (bulkUpdateOps.length > 0) db.coll.bulkWrite(bulkUpdateOps);
Or for MongoDB 2.6.x and 3.0.x releases, use this version of Bulk operations:
var bulk = db.coll.initializeUnorderedBulkOp(),
counter = 0;
db.coll.aggregate([
{ "$match": { "users": { "$exists": true } } }
{
"$project": {
"computed_field": {
"$divide": ["$users", 100]
}
}
}
]).forEach(function(doc) {
bulk.find({ "_id": doc._id })
.updateOne({ "$set": { "users": doc.computed_field } });
if (counter % 500 === 0) {
bulk.execute();
bulk = db.coll.initializeUnorderedBulkOp();
}
});
if (counter % 500 !== 0 ) bulk.execute();
The Bulk operations API in both cases will help reduce the IO load on the server by sending the requests only once in every 500 documents in the collection to process.