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.

Answer 1

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")
            )
        );
    }
});