Random documents from MongoDB using spring-data

Question

I\'m able to do it by using this mongodb native query:

db.books.aggregate(
   [ { $sample: { size: 15 } } ]
)

But how to do it in spr

Answer 1

Blakes Seven answered it correctly, however, I want to offer a nicer implementation of AggregationOperation, which follows standard Spring implementation

import com.mongodb.BasicDBObject;
import com.mongodb.DBObject;
import org.springframework.data.mongodb.core.aggregation.AggregationOperation;
import org.springframework.data.mongodb.core.aggregation.AggregationOperationContext;
import org.springframework.data.mongodb.core.query.Criteria;
import org.springframework.util.Assert;

public class SampleOperation implements AggregationOperation  {

    private int size;

    public SampleOperation(int size) {
        Assert.isTrue(size > 0, " Size must be positive!");
        this.size = size;
    }

    public AggregationOperation setSize(int size) {
        Assert.isTrue(size > 0, " Size must be positive!");
        this.size = size;
        return this;
    }

    @Override
    public DBObject toDBObject(AggregationOperationContext context) {
        return new BasicDBObject("$sample", context.getMappedObject(Criteria.where("size").is(size).getCriteriaObject()));
    }

}

After that, you can create SampleOperation object with constructor, or later change the size of it by setSize() method, and apply it to aggregate() function as normal.

Update: In SpringBoot 2.0.0+ and Spring Framework 5.0: Spring Mongo drop DBObject and replace by org.bson.Document therefore the last past should be updated as:

    @Override
public Document toDocument(AggregationOperationContext aggregationOperationContext) {
    return new Document("$sample", aggregationOperationContext.getMappedObject(Criteria.where("size").is(size).getCriteriaObject()));

}

And remove the @Override toDBObject