MongoDB how to count array elements with Java driver

Question

I need my application to count the size of an array foos inside a document using Java driver 3.8. I know I can do this with the shell

db.collection.aggregate(
   [
      {
         $match: { _id: 123456789 }
      },
      {
         $project: {
            count: { $size: "$foos" }
         }
      }
   ]
)

but I have no idea of how to do this with java, because the only method producing a $size operator I found is Filters.size(String fieldName, int size) that is meant to look for documents where array fieldName has size size. I searched inside com.mongodb.client.model package but I found nothing to answer my question.

s7vr · Accepted Answer · 2018-08-08 18:37:22Z

5

No there are no helpers in java driver to help with aggregation expression for $project stage.

Try

Bson match = new Document("$match", Filters.eq("_id", 123456789 ));
Bson projection = new Document("$size", "$foos" );
Bson project = Aggregates.project(new Document("count", projection) );
collection.aggregate(Arrays.asList(match, project));

answered Aug 8, 2018 at 18:37

s7vr

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Comments

Bathri Nathan · Accepted Answer · 2020-10-30 14:20:53Z

0

You can create a new field and put the size of the array inside it and use it for count or sorting.

use "addFields" java Helper

addFields(new Field("productsCount",
            new Document("$size", "$products")));

Where

'productsCount' is the name of the new field
'products' is the Array
'$size' is the MongoDB aggregation.

answered Oct 30, 2020 at 14:20

Bathri Nathan

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1 Comment

Michael Over a year ago

Then you have to maintain this field. Saving calculations is almost never a good idea. (Notable exception being long calculations) It's better to calculate in memory.

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