How to write redact aggregation in java?

问题

I'm trying to convert MongoDb query using aggregation framework using Java driver. I have been helped to create query here How to apply filter for output of aggregation framework of Mongo Db?.

Here is the sample aggregate query:

db.movies.aggregate(
[{
    $redact: {
        $cond: {
            if: {$gt: [{ $avg: "$customerReviews"}, 7 ] },
            then: "$$KEEP",
            else: "$$PRUNE"
        }
    }
},
{$skip:1},
{$limit:2}
]
);

I started with:

BasicDBObject avgQuery = new BasicDBObject("$avg", "$customerReviews");

But cannot figure out how to perform {$gt: [{ $avg: "$customerReviews"}, 7 ] }. I think it should be something like gtQuery.put(avgQuery, new BasicDbObject("$gt",7)) but obviously cannot put something other than String in put() function.

Btw, I'm not sure if $redact can be done only using BasicDbObject or I need something like Mongo spring query where two fields are equal which uses Spring Mongo. Hope someone can help me get through the whole query.

回答1:

BasicDBObject is old 2.x mongo version classes. Use the newer 3.x api classes.

I don't see any helper class to create redact pipeline in the java driver.

MongoClient mongoClient = new MongoClient();
MongoDatabase database = mongoClient.getDatabase("dbname");
MongoCollection<Document> collection = database.getCollection("dbcollection");
List<Document> results = collection.aggregate(Arrays.asList(
            new Document("$redact", new Document("$cond",
                    Arrays.asList(new Document("$gt",
                    Arrays.asList(new Document("$avg", "$customerReviews"), 7)), 
                    "$$KEEP", "$$PRUNE"))),
            Aggregates.skip(1),
            Aggregates.limit(2)
)).into(new ArrayList<>());

Alternatively you can use

String redact = "{\n" +
            "    $redact: {\n" +
            "        $cond: {\n" +
            "            if: {$gt: [{ $avg: \"$customerReviews\"}, 7 ] },\n" +
            "            then: \"$$KEEP\",\n" +
            "            else: \"$$PRUNE\"\n" +
            "        }\n" +
            "    }\n" +
            "}";

List<Document> results = collection.aggregate(Arrays.asList(
            Document.parse(redact),
            Aggregates.skip(1),
            Aggregates.limit(2)
)).into(new ArrayList<>());

来源：https://stackoverflow.com/questions/42357768/how-to-write-redact-aggregation-in-java

标签

mongodb

aggregation-framework

mongo-java-driver