How to Update Multiple Array Elements in mongodb

Question

UPDATE: As of Mongo version 3.6, this answer is no longer valid as the mentioned issue was fixed and there are ways to achieve this. Please check other answers.

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At this moment it is not possible to use the positional operator to update all items in an array. See JIRA http://jira.mongodb.org/browse/SERVER-1243

As a work around you can:

• Update each item individually (events.0.handled events.1.handled ...) or...
• Read the document, do the edits manually and save it replacing the older one (check "Update if Current" if you want to ensure atomic updates)

With the release of MongoDB 3.6 ( and available in the development branch from MongoDB 3.5.12 ) you can now update multiple array elements in a single request.

This uses the filtered positional $[<identifier>] update operator syntax introduced in this version:

db.collection.update(
  { "events.profile":10 },
  { "$set": { "events.$[elem].handled": 0 } },
  { "arrayFilters": [{ "elem.profile": 10 }], "multi": true }
)

The "arrayFilters" as passed to the options for .update() or even .updateOne(), .updateMany(), .findOneAndUpdate() or .bulkWrite() method specifies the conditions to match on the identifier given in the update statement. Any elements that match the condition given will be updated.

Noting that the "multi" as given in the context of the question was used in the expectation that this would "update multiple elements" but this was not and still is not the case. It's usage here applies to "multiple documents" as has always been the case or now otherwise specified as the mandatory setting of .updateMany() in modern API versions.

NOTE Somewhat ironically, since this is specified in the "options" argument for .update() and like methods, the syntax is generally compatible with all recent release driver versions.

However this is not true of the mongo shell, since the way the method is implemented there ( "ironically for backward compatibility" ) the arrayFilters argument is not recognized and removed by an internal method that parses the options in order to deliver "backward compatibility" with prior MongoDB server versions and a "legacy" .update() API call syntax.

So if you want to use the command in the mongo shell or other "shell based" products ( notably Robo 3T ) you need a latest version from either the development branch or production release as of 3.6 or greater.

See also positional all $[] which also updates "multiple array elements" but without applying to specified conditions and applies to all elements in the array where that is the desired action.

Also see Updating a Nested Array with MongoDB for how these new positional operators apply to "nested" array structures, where "arrays are within other arrays".

IMPORTANT - Upgraded installations from previous versions "may" have not enabled MongoDB features, which can also cause statements to fail. You should ensure your upgrade procedure is complete with details such as index upgrades and then run

   db.adminCommand( { setFeatureCompatibilityVersion: "3.6" } )

Or higher version as is applicable to your installed version. i.e "4.0" for version 4 and onwards at present. This enabled such features as the new positional update operators and others. You can also check with:

   db.adminCommand( { getParameter: 1, featureCompatibilityVersion: 1 } )

To return the current setting

What worked for me was this:

db.collection.find({ _id: ObjectId('4d2d8deff4e6c1d71fc29a07') })
  .forEach(function (doc) {
    doc.events.forEach(function (event) {
      if (event.profile === 10) {
        event.handled=0;
      }
    });
    db.collection.save(doc);
  });

I think it's clearer for mongo newbies and anyone familiar with JQuery & friends.

This can also be accomplished with a while loop which checks to see if any documents remain that still have subdocuments that have not been updated. This method preserves the atomicity of your updates (which many of the other solutions here do not).

var query = {
    events: {
        $elemMatch: {
            profile: 10,
            handled: { $ne: 0 }
        }
    }
};

while (db.yourCollection.find(query).count() > 0) {
    db.yourCollection.update(
        query,
        { $set: { "events.$.handled": 0 } },
        { multi: true }
    );
}

The number of times the loop is executed will equal the maximum number of times subdocuments with profile equal to 10 and handled not equal to 0 occur in any of the documents in your collection. So if you have 100 documents in your collection and one of them has three subdocuments that match query and all the other documents have fewer matching subdocuments, the loop will execute three times.

This method avoids the danger of clobbering other data that may be updated by another process while this script executes. It also minimizes the amount of data being transferred between client and server.

This does in fact relate to the long standing issue at http://jira.mongodb.org/browse/SERVER-1243 where there are in fact a number of challenges to a clear syntax that supports "all cases" where mutiple array matches are found. There are in fact methods already in place that "aid" in solutions to this problem, such as Bulk Operations which have been implemented after this original post.

It is still not possible to update more than a single matched array element in a single update statement, so even with a "multi" update all you will ever be able to update is just one mathed element in the array for each document in that single statement.

The best possible solution at present is to find and loop all matched documents and process Bulk updates which will at least allow many operations to be sent in a single request with a singular response. You can optionally use .aggregate() to reduce the array content returned in the search result to just those that match the conditions for the update selection:

db.collection.aggregate([
    { "$match": { "events.handled": 1 } },
    { "$project": {
        "events": {
            "$setDifference": [
               { "$map": {
                   "input": "$events",
                   "as": "event",
                   "in": {
                       "$cond": [
                           { "$eq": [ "$$event.handled", 1 ] },
                           "$$el",
                           false
                       ]
                   }
               }},
               [false]
            ]
        }
    }}
]).forEach(function(doc) {
    doc.events.forEach(function(event) {
        bulk.find({ "_id": doc._id, "events.handled": 1  }).updateOne({
            "$set": { "events.$.handled": 0 }
        });
        count++;

        if ( count % 1000 == 0 ) {
            bulk.execute();
            bulk = db.collection.initializeOrderedBulkOp();
        }
    });
});

if ( count % 1000 != 0 )
    bulk.execute();

The .aggregate() portion there will work when there is a "unique" identifier for the array or all content for each element forms a "unique" element itself. This is due to the "set" operator in $setDifference used to filter any false values returned from the $map operation used to process the array for matches.

If your array content does not have unique elements you can try an alternate approach with $redact:

db.collection.aggregate([
    { "$match": { "events.handled": 1 } },
    { "$redact": {
        "$cond": {
            "if": {
                "$eq": [ { "$ifNull": [ "$handled", 1 ] }, 1 ]
            },
            "then": "$$DESCEND",
            "else": "$$PRUNE"
        }
    }}
])

Where it's limitation is that if "handled" was in fact a field meant to be present at other document levels then you are likely going to get unexepected results, but is fine where that field appears only in one document position and is an equality match.

Future releases ( post 3.1 MongoDB ) as of writing will have a $filter operation that is simpler:

db.collection.aggregate([
    { "$match": { "events.handled": 1 } },
    { "$project": {
        "events": {
            "$filter": {
                "input": "$events",
                "as": "event",
                "cond": { "$eq": [ "$$event.handled", 1 ] }
            }
        }
    }}
])

And all releases that support .aggregate() can use the following approach with $unwind, but the usage of that operator makes it the least efficient approach due to the array expansion in the pipeline:

db.collection.aggregate([
    { "$match": { "events.handled": 1 } },
    { "$unwind": "$events" },
    { "$match": { "events.handled": 1 } },
    { "$group": {
        "_id": "$_id",
        "events": { "$push": "$events" }
    }}        
])

In all cases where the MongoDB version supports a "cursor" from aggregate output, then this is just a matter of choosing an approach and iterating the results with the same block of code shown to process the Bulk update statements. Bulk Operations and "cursors" from aggregate output are introduced in the same version ( MongoDB 2.6 ) and therefore usually work hand in hand for processing.

In even earlier versions then it is probably best to just use .find() to return the cursor, and filter out the execution of statements to just the number of times the array element is matched for the .update() iterations:

db.collection.find({ "events.handled": 1 }).forEach(function(doc){ 
    doc.events.filter(function(event){ return event.handled == 1 }).forEach(function(event){
        db.collection.update({ "_id": doc._id },{ "$set": { "events.$.handled": 0 }});
    });
});

If you are aboslutely determined to do "multi" updates or deem that to be ultimately more efficient than processing multiple updates for each matched document, then you can always determine the maximum number of possible array matches and just execute a "multi" update that many times, until basically there are no more documents to update.

A valid approach for MongoDB 2.4 and 2.2 versions could also use .aggregate() to find this value:

var result = db.collection.aggregate([
    { "$match": { "events.handled": 1 } },
    { "$unwind": "$events" },
    { "$match": { "events.handled": 1 } },
    { "$group": {
        "_id": "$_id",
        "count": { "$sum": 1 }
    }},
    { "$group": {
        "_id": null,
        "count": { "$max": "$count" }
    }}
]);

var max = result.result[0].count;

while ( max-- ) {
    db.collection.update({ "events.handled": 1},{ "$set": { "events.$.handled": 0 }},{ "multi": true })
}

Whatever the case, there are certain things you do not want to do within the update:

1. Do not "one shot" update the array: Where if you think it might be more efficient to update the whole array content in code and then just $set the whole array in each document. This might seem faster to process, but there is no guarantee that the array content has not changed since it was read and the update is performed. Though $set is still an atomic operator, it will only update the array with what it "thinks" is the correct data, and thus is likely to overwrite any changes occurring between read and write.

2. Do not calculate index values to update: Where similar to the "one shot" approach you just work out that position 0 and position 2 ( and so on ) are the elements to update and code these in with and eventual statement like:

   { "$set": {
       "events.0.handled": 0,
       "events.2.handled": 0
   }}
   

   Again the problem here is the "presumption" that those index values found when the document was read are the same index values in th array at the time of update. If new items are added to the array in a way that changes the order then those positions are not longer valid and the wrong items are in fact updated.

So until there is a reasonable syntax determined for allowing multiple matched array elements to be processed in single update statement then the basic approach is to either update each matched array element in an indvidual statement ( ideally in Bulk ) or essentially work out the maximum array elements to update or keep updating until no more modified results are returned. At any rate, you should "always" be processing positional $ updates on the matched array element, even if that is only updating one element per statement.

Bulk Operations are in fact the "generalized" solution to processing any operations that work out to be "multiple operations", and since there are more applications for this than merely updating mutiple array elements with the same value, then it has of course been implemented already, and it is presently the best approach to solve this problem.