java mongodb - 获取数组长度而不下载所有数据

Question

I'm using mongodb to store data for my java program and I have a collection with an array field that has a lot of things in it but i want only to get the length, without all the other data.

Now i'm using this to get it:

((UUID[])document.get("customers")).length

How can I make this not to download all the array?

A possible answer is to create an int that counts the pushes and the pulls of the array but it's not the cleanest method.

Answer 1

You are looking for aggregation framework where you can use the $size operator in your pipeline, this counts and returns the total the number of items in an array:

db.collection.aggregate([
    {
        "$project": {
            "_id": 0, "customer_count": { "$size": "$customers" }
         }
    }
]);

where the Java equivalent:

DBObject projectFields = new BasicDBObject("_id", 0);
projectFields.put("customer_count", new BasicDBObject( "$size", "$customers" ));
DBObject project = new BasicDBObject("$project", projectFields);

AggregationOutput output = db.getCollection("collectionName").aggregate(project);

System.out.println("\n" + output);

Answer 2

You can use MongoDB's Aggregation Framework to get the size of the array. For example, given the following document structure:

> db.macross.findOne()
{
    "_id" : "SDF1",
    "crew" : [
            "Rick",
            "Minmay",
            "Roy",
            "Max",
            "Misa",
            "Milia"
    ]
}

get the size of the array

> db.macross.aggregate( 
    { $match: { _id: "SDF1" } }, 
    { $unwind: "$crew" }, 
    { $group: { _id: "", count: { $sum: 1 } } }, 
    { $project: { _id: 0, count: 1 } }
)
{ "count" : 6 }

More detailed and interesting examples are available in the docs .