MongoDB Java - Create a new ObjectId for each element of an existing array

Question

I have an existing collection, containing several documents.

[{
    "_id": "...1",
    "prop1": "...",
    "prop2": "...",
    "someArray": [
        {
            "value": "sub element 1.1"
        },
        {
            "value": "sub element 1.2"
        },
        {
            "value": "sub element 1.3"
        }
    ]
}, {
    "_id": "...2",
    "prop1": "...",
    "prop2": "...",
    "someArray": [
        {
            "value": "sub element 2.1"
        },
        {
            "value": "sub element 2.2"
        }
    ]
}, // many others here...
]

For each root document, I would like to add an _id property of type ObjectId on each sub-element of someArray. So, after I run my command, the content of the collection is the following:

[{
    "_id": "...1",
    "prop1": "...",
    "prop2": "...",
    "someArray": [
        {
            "_id": ObjectId("..."),
            "value": "sub element 1.1"
        },
        {
            "_id": ObjectId("..."),
            "value": "sub element 1.2"
        },
        {
            "_id": ObjectId("..."),
            "value": "sub element 1.3"
        }
    ]
}, {
    "_id": "...2",
    "prop1": "...",
    "prop2": "...",
    "someArray": [
        {
            "_id": ObjectId("..."),
            "value": "sub element 2.1"
        },
        {
            "_id": ObjectId("..."),
            "value": "sub element 2.2"
        }
    ]
}, // ...
]

Each ObjectId being, of course, unique.

The closer I got was with this:

db.getCollection('myCollection').updateMany({}, { "$set" : { "someArray.$[]._id" : ObjectId() } });

But every sub-element of the entire collection ends up with the same ObjectId value...

Ideally, I need to get this working using Java driver for MongoDB. The closest version I got is this (which presents the exact same problem: all the ObjectId created have the same value).

database
    .getCollection("myCollection")
    .updateMany(
        Filters.ne("someArray", Collections.emptyList()), // do not update empty arrays
        new Document("$set", new Document("someArray.$[el]._id", "ObjectId()")), // set the new ObjectId...
        new UpdateOptions().arrayFilters(
            Arrays.asList(Filters.exists("el._id", false)) // ... only when the _id property doesn't already exist
        )
    );

ray · Accepted Answer · 2022-04-19 21:14:04Z

2

With MongoDB v4.4+, you can use $function to use javascript to assign the _id in the array.

db.collection.aggregate([
  {
    "$addFields": {
      "someArray": {
        $function: {
          body: function(arr) {
                  return arr.map(function(elem) {
                           elem['_id'] = new ObjectId(); 
                           return elem;
                         })
                },
          args: [
            "$someArray"
          ],
          lang: "js"
        }
      }
    }
  }
])

Here is the Mongo playground for your reference. (It's slightly different from the code above as playground requires the js code to be in double quote)

For older version of MongoDB, you will need to use javascript to loop the documents and update them one by one.

db.getCollection("...").find({}).forEach(function(doc) {
  doc.someArray = doc.someArray.map(function(elem) {
    elem['_id'] = new ObjectId(); 
    return elem;
  })
  db.getCollection("...").save(doc);
})

edited Apr 19, 2022 at 21:14

answered Apr 19, 2022 at 20:57

ray

15.6k12 gold badges96 silver badges104 bronze badges

Sign up to request clarification or add additional context in comments.

6 Comments

Eria Over a year ago

Thanks for the answer, but db.version() returns 3.6.21 :/

ray Over a year ago

@Eria updated the answer to include the older way of updating documents in MongoDB. Please have a try and see if it works for you.

Eria Over a year ago

I can't use this as-is in Java. The Javascript part does not compile. I will try to find a way to adapt it.

ray Over a year ago

@Eria you dont need to use this in java. You just need to open a mongo shell and execute the script. Check out Mongo shell. It should come natively with your MongoDB.

Eria Over a year ago

Ultimately, I have to put this in a mongock change log, integrated in our app source code. So, yes, I need to translate this in Java ^^'

|

Eria · Accepted Answer · 2022-05-27 20:46:56Z

Here is what I managed to write in the end:

MongoCollection<Document> collection = database.getCollection("myCollection");
collection
    .find(Filters.ne("someArray", Collections.emptyList()), MyItem.class)
    .forEach(item -> {
        item.getSomeArray().forEach(element -> {
            if( element.getId() == null ){
                collection.updateOne(
                    Filters.and(
                        Filters.eq("_id", item.getId()),
                        Filters.eq("someArray.value", element.getValue())
                    ),
                    Updates.set("someArray.$._id", new ObjectId())
                );
            }
        });
    });

The value property of sub-elements had to be unique (and luckily it was). And I had to perform separate updateOne operations in order to obtain a different ObjectId for each element.

Collectives™ on Stack Overflow

MongoDB Java - Create a new ObjectId for each element of an existing array

2 Answers 2

6 Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

6 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related