Project using array fields in MongoDB Java Driver

Question

I have Collection as following. I have query this collection by user.

 {
      "user": "username",
      "sites": {
        "site": "abc",
        "keywords": [
          {
            "keyword": "keyword1",
            "dailyranks": [
              {
                "fild1": "value1"
              },
              {
                "fild2": "value2"
              },
              {
                "fild3": "value3"
              },
            ]
          },
          {
            "keyword": "keyword2",
            "dailyranks": [
              {
                "fild1": "value1"
              },
              {
                "fild2": "value2"
              },
              {
                "fild3": "value3"
              },  
            ]
          },
        ],
      }
    }

I want to get the result from the collection as follows, I want to get the last elements of keyword array in the collection

[
    {
        "keyword" : "keyword1"
        "fild2" : "value2",
        "fild3" : "value3"
    },
    {
       "keyword" : "keyword2"
        "fild2" : "value2",
        "fild3" : "value3"
    },
]

I have aggregate using $project, but didn't work out. help me to sort this out.

Code used-

BasicDBObject siteObject = new BasicDBObject();
siteObject.append("keywords", "$sites.keywords.keyword");
siteObject.append("lastrank", "$sites.keywords.dailyranks");

BasicDBList aDBList = new BasicDBList();
aaa.add(new BasicDBObject("user", modelLogin.getUSER_NAME()));

ArrayList<BasicDBObject> doc = new ArrayList<>();
doc.add(new BasicDBObject().append("$unwind", "$sites"));
doc.add(new BasicDBObject("$match", aDBList));
 doc.add(new BasicDBObject().append("$project", siteObject));
AggregationOutput output = coll.aggregate(doc);

Can you show what you've tried so far, with your aggregation and $project? — Vince Bowdren
– Vince Bowdren, Commented Mar 17, 2017 at 10:14

s7vr · Accepted Answer · 2017-03-20 07:12:04Z

2

You can try below aggregation.

Use $map to transform the Keywords array.

Within $map, use $arrayElemAt to project the last and second last values from dailyranks and $let operator to hold the result from $arrayAtElem and project the fild value.

db.coll.aggregate({
    $project: {
        keywords: {
            $map: {
                input: "$sites.keywords",
                as: "result",
                in: {
                    keyword: "$$result.keyword",
                    fild2: {$let: {vars: {obj: {$arrayElemAt: ["$$result.dailyranks", -2]}},in: "$$obj.fild2"}},
                    fild3: {$let: {vars: {obj: {$arrayElemAt: ["$$result.dailyranks", -1]}},in: "$$obj.fild3"}}
                }
            }
        }
    }
})

Java Equivalent

MongoClient mongoClient = new MongoClient();
MongoDatabase db  = mongoClient.getDatabase("db")
MongoCollection<Document> collection = db.getCollection("collection");
List<Document> results =
    collection.aggregate(
        Arrays.asList(
            Aggregates.match(Filters.eq("user", modelLogin.getUSER_NAME() )),
            Aggregates.project(
                Projections.fields(
                    new Document("keywords",
                        new Document("$map",
                        new Document("input", "$sites.keywords").
                        append("as", "result").
                        append("in",
                            new Document("keyword","$$result.keyword").
                                append("fild2",
                                    new Document("$let",
                                    new Document("vars", new Document("obj",
                                            new Document("$arrayElemAt", Arrays.asList("$$result.dailyranks", -2)))).
                                    append("in", "$$obj.fild2" ))).
                                append("fild3",
                                    new Document("$let",
                                    new Document("vars", new Document("obj",
                                            new Document("$arrayElemAt", Arrays.asList("$$result.dailyranks", -1)))).
                                    append("in", "$$obj.fild3" )))
                            )
                        )
                    )))
        )).into(new ArrayList<>());

edited Mar 20, 2017 at 7:12

answered Mar 17, 2017 at 13:40

s7vr

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9 Comments

Amal Madawa Over a year ago

@veerram what is the purpose of " fields" in Aggregates.project(). got comping error

s7vr Over a year ago

It's use to project the values. Updated. More here mongodb.github.io/mongo-java-driver/3.4/javadoc/?com/mongodb/…

Amal Madawa Over a year ago

got compile error like this, no suitable method found for aggregate(List<Bson>) method DBCollection.aggregate(DBObject,DBObject...) is not applicable

s7vr Over a year ago

What is your mongo java driver version ?

Amal Madawa Over a year ago

it is 3.3.0. I just update it to 3.4 but still getting the same error

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