How to limit number of updating documents in mongodb

How to implement somethings similar to db.collection.find().limit(10) but while updating documents?

Now I'm using something r开发者_运维技巧eally 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.

---

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} );

---

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>})

---

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.

---

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).

---

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>})})

---

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>})
}

---

    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"}
    })

---

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.