How to limit number of updating documents in mongodb - mongodb

Question

How to implement somethings similar to db.collection.find().limit(10) but while updating documents?
Now I'm using something really crappy like getting documents with db.collection.find().limit() and then updating them.
In general I wanna to return given number of records and change one field in each of them.
Thanks.

Answer 1

You can use:
db.collection.find().limit(NUMBER_OF_ITEMS_YOU_WANT_TO_UPDATE).forEach(
function (e) {
e.fieldToChange = "blah";
....
db.collection.save(e);
}
);
(Credits for forEach code: MongoDB: Updating documents using data from the same document)
What this will do is only change the number of entries you specify. So if you want to add a field called "newField" with value 1 to only half of your entries inside "collection", for example, you can put in
db.collection.find().limit(db.collection.count() / 2).forEach(
function (e) {
e.newField = 1;
db.collection.save(e);
}
);
If you then want to make the other half also have "newField" but with value 2, you can do an update with the condition that newField doesn't exist:
db.collection.update( { newField : { $exists : false } }, { $set : { newField : 2 } }, {multi : true} );

Answer 2

Using forEach to individually update each document is slow. You can update the documents in bulk using
ids = db.collection.find(<condition>).limit(<limit>).map(
function(doc) {
return doc._id;
}
);
db.collection.updateMany({_id: {$in: ids}}, <update>})

Answer 3

The solutions that iterate over all objects then update them individually are very slow.
Retrieving them all then updating simultaneously using $in is more efficient.
ids = People.where(firstname: 'Pablo').limit(10000).only(:_id).to_a.map(&:id)
People.in(_id: ids).update_all(lastname: 'Cantero')
The query is written using Mongoid, but can be easily rewritten in Mongo Shell as well.

Answer 4

Unfortunately the workaround you have is the only way to do it AFAIK. There is a boolean flag multi which will either update all the matches (when true) or update the 1st match (when false).

Answer 5

As the answer states there is still no way to limit the number of documents to update (or delete) to a value > 1. A workaround to use something like:
db.collection.find(<condition>).limit(<limit>).forEach(function(doc){db.collection.update({_id:doc._id},{<your update>})})

Answer 6

If your id is a sequence number and not an ObjectId you can do this in a for loop:
let batchSize= 10;
for (let i = 0; i <= 1000000; i += batchSize) {
db.collection.update({$and :[{"_id": {$lte: i+batchSize}}, {"_id": {$gt: i}}]}),{<your update>})
}

Answer 7

let fetchStandby = await db.model.distinct("key",{});
fetchStandby = fetchStandby.slice(0, no_of_docs_to_be_updated)
let fetch = await db.model.updateMany({
key: { $in: fetchStandby }
}, {
$set:{"qc.status": "pending"}
})

Answer 8

I also recently wanted something like this. I think querying for a long list of _id just to update in an $in is perhaps slow too, so I tried to use an aggregation+merge
while (true) {
const record = db.records.findOne({ isArchived: false }, {_id: 1})
if (!record) {
print("No more records")
break
}
db.records.aggregate([
{ $match: { isArchived: false } },
{ $limit: 100 },
{
$project: {
_id: 1,
isArchived: {
$literal: true
},
updatedAt: {
$literal: new Date()
}
}
},
{
$merge: {
into: "records",
on: "_id",
whenMatched: "merge"
}
}
])
print("Done update")
}
But feel free to comment if this is better or worse that a bulk update with $in.