How to filter documents based on an embedded array?

Question

After reviewing this page, specifically this query

db.scores.find(
   { results: { $elemMatch: { $gte: 80, $lt: 85 } } }
)

I used the following imports

import static com.mongodb.client.model.Filters.and;
import static com.mongodb.client.model.Filters.elemMatch;
import static com.mongodb.client.model.Filters.eq;
import static com.mongodb.client.model.Projections.excludeId;
import static com.mongodb.client.model.Projections.fields;
import static com.mongodb.client.model.Projections.include;

And came up with the following code to perform a similar operation (ARRAY_FIELD_NAME = "myArray")

MongoCollection<Document> collection = mongoDB.getCollection(COLLECTION_NAME);

Bson filters = and(eq("userId", userId), elemMatch(ARRAY_FIELD_NAME, eq("id", id)));
Bson projections = fields(include(ARRAY_FIELD_NAME), excludeId());

List<Document> results = (List<Document>) collection.find(filters).projection(projections).first().get(ARRAY_FIELD_NAME);
if (CollectionUtils.isEmpty(results)) {
    return null;
}
if (results.size() > 1) {
    throw new ApiException(String.format("Multiple results matched (User ID: %s, Array item ID: %s)", userId, id));
}
return results.get(0);

To filter documents that have the following structure

{
    "_id": {
        "$oid": "588899721bbabc26865f41cc"
    },
    "userId": 55,
    "myArray": [
        {
            "id": "5888998e1bbabc26865f41d2",
            "title": "ABC"
        },
        {
            "id": "5888aaf41bbabc3200e252aa",
            "title": "ABC"
        }
    ]
}

But instead of getting a single or no item from the myArray field, I always get both items !

The only code that worked for me is the following

MongoCollection<Document> collection = mongoDB.getCollection(COLLECTION_NAME);

List<Bson> aggregationFlags = new ArrayList<>();
aggregationFlags.add(new Document("$unwind", "$" + ARRAY_FIELD_NAME));
aggregationFlags.add(new Document("$match", new Document("userId", userId).append(ARRAY_FIELD_NAME + ".id", id)));
aggregationFlags.add(new Document("$project", new Document("_id", 0).append(ARRAY_FIELD_NAME, "$" + ARRAY_FIELD_NAME)));

return (Document) collection.aggregate(aggregationFlags).first().get(ARRAY_FIELD_NAME);

So why does the first piece of code that should behave the same as the query shown at the beginning of the question, not filter results as expected ?

I do not need to "aggregate" results, I need to "filter" them using the user ID and array item id.

s7vr · Accepted Answer · 2017-01-25 15:19:04Z

1

You need to use $elemMatch(projection). Something like below should work.

import static com.mongodb.client.model.Projections.elemMatch;

Bson filters = and(eq("userId", userId));
Bson projections = fields(elemMatch(ARRAY_FIELD_NAME, eq("id", id)), excludeId());

answered Jan 25, 2017 at 15:19

s7vr

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1 Comment

Muhammad Gelbana Over a year ago

At first, I couldn't see how your answers addresses my problem. But I kept reading the elemMatch documentation for projections and filters and only now I understand. Filters.elemMatch filters the documents with the matching array filter. While Projections.elemMatch, limits the document's array items ! This was very confusing !

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