MongoDB Java - Update every sub-array element using another field value

Question

I have an existing collection, containing several documents:

[{
    "_id": "1",
    "someArray": [
        {
            "_id": "1.1"
            "color": "RED"
        },
        {
            "_id": "1.2"
            "color": "GREEN"
        },
        {
            "_id": "1.3"
            "color": "GREEN"
        }
    ]
}, {
    "_id": "2",
    "someArray": [
        {
            "_id": "2.1"
            "color": "WHITE"
        },
        {
            "_id": "2.2"
            "color": "BLUE"
        }
    ]
}, // many others here...
]

I need to replace the color field of the sub-elements by a colors field, which is an array containing the same value color did.

Here is the result I want to obtain:

[{
    "_id": "1",
    "someArray": [
        {
            "_id": "1.1"
            "colors": [ "RED" ]
        },
        {
            "_id": "1.2"
            "colors": [ "GREEN" ]
        },
        {
            "_id": "1.3"
            "colors": [ "GREEN" ]
        }
    ]
}, {
    "_id": "2",
    "someArray": [
        {
            "_id": "2.1"
            "colors": [ "WHITE" ]
        },
        {
            "_id": "2.2"
            "colors": [ "BLUE" ]
        }
    ]
}]

My closest attempt was this:

collection.updateMany(
    Filters.ne("someArray", Collections.emptyList()),
    Updates.combine(
        Updates.set("someArray.$[].colors", "[ $someArray.color ]"),
        Updates.unset("someArray.$[].color")
    )
);

But the value of colors is inserted as a String, as-is. It's not interpreted as "an array containing the value of color field".

This has to be done in Java. Please don't provide JS-based solution.

you can not rename it by regular update query, use update with aggregation pipeline — turivishal
– turivishal, Commented May 31, 2022 at 16:40

Eria · Accepted Answer · 2022-06-01 12:44:32Z

The solution I finally came up with...

MongoCollection<Document> collection = database.getCollection("myCollection");
collection.find(
    Filters.ne("someArray", Collections.emptyList()), MyDocumentRoot.class
).forEach(rootElement -> {
    for(int i = 0; i < rootElement.getSomeArray().size(); i++){
        Document document = collection.find(Filters.eq("_id", rootElement.getId())).first();
        String color = document.getList("someArray", Document.class)
            .get(i)
            .getString("color");
        collection.updateOne(
            Filters.and(
                Filters.eq("_id", rootElement.getId()),
                Filters.eq("someArray._id", rootElement.getSomeArray().get(i).getId())
            ),
            Updates.combine(
                Updates.set("someArray.$.colors", Collections.singleton(color)),
                Updates.unset("someArray.$.color")
            )
        );
    }
});

Charchit Kapoor · Accepted Answer · 2022-06-01 12:34:32Z

As suggested in the comment section, I am afraid this can be done with a simple update query, however, if you are using MongoDB version 4.2 and above, you can use $merge to your advantage, to generate your updated document and merge it in the existing collection like this:

MongoClient mongoClient = new MongoClient(
                new MongoClientURI(
                        "mongodb://localhost:27017/"
                )
        );
        MongoDatabase database = mongoClient.getDatabase("StackOverflow");
        MongoCollection<Document> collection = database.getCollection("Sample");

        FindIterable<Document> result = (FindIterable<Document>) collection.aggregate(Arrays.asList(new Document("$match",
                        new Document("someArray",
                                new Document("$exists", true)
                                        .append("$ne", Arrays.asList()))),
                new Document("$unwind",
                        new Document("path", "$someArray")),
                new Document("$set",
                        new Document("someArray.colors", Arrays.asList("$someArray.color"))),
                new Document("$unset", "someArray.color"),
                new Document("$group",
                        new Document("_id", "$_id")
                                .append("someArray",
                                        new Document("$push", "$someArray"))),
                new Document("$merge", "collection")));

Here is the working, mongo shell equivalent pipeline.

Thanks for your answer. Unfortunately, I'm using MongoDB 3.6 (for now). In the meantime, I found a Java-based solution. I will publish it now.

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