来自MongoDB的随机文档使用spring-data

Question

我可以通过使用这个mongodb本机查询来做到这一点:

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

但是怎么做呢spring-data-mongodb？

我在Spring Aggregation Framework的Aggregation类中找不到类似的操作

Answer 1

更新:

从Spring Data v2.0开始,您可以这样做:

SampleOperation matchStage = Aggregation.sample(5);
Aggregation aggregation = Aggregation.newAggregation(sampleStage);
AggregationResults<OutType> output = mongoTemplate.aggregate(aggregation, "collectionName", OutType.class);

原始答案:

像spring-mongo这样的抽象层总是落后于服务器发布的功能.因此,您最好自己为流水线阶段构建BSON文档结构.

在自定义类中实现:

public class CustomAggregationOperation implements AggregationOperation {
    private DBObject operation;

    public CustomAggregationOperation (DBObject operation) {
        this.operation = operation;
    }

    @Override
    public DBObject toDBObject(AggregationOperationContext context) {
        return context.getMappedObject(operation);
    }
}

然后在你的代码中使用:

Aggregation aggregation = newAggregation(
    new CutomAggregationOperation(
        new BasicDBObject(
            "$sample",
            new BasicDBObject( "size", 15 )
        )
    )
);

由于这实现了AggregationOperation这适用于exising管道操作辅助方法.即:

Aggregation aggregation = newAggregation(
    // custom pipeline stage
    new CutomAggregationOperation(
        new BasicDBObject(
            "$sample",
            new BasicDBObject( "size", 15 )
        )
    ),
    // Standard match pipeline stage
    match(
        Criteria.where("myDate")
            .gte(new Date(new Long("949384052490")))
            .lte(new Date(new Long("1448257684431")))
    )
);

所以,在一天结束时,一切都只是一个BSON对象.这只是一个接口包装器的问题,以便spring-mongo中的类方法解释结果并正确获取您定义的BSON对象.