问题描述

我可以通过使用这个 mongodb 本机查询来做到这一点:

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

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

但是如何在 spring-data-mongodb 中做到呢?

But how to do it in spring-data-mongodb ?

聚合类="nofollow">Spring 聚合框架

I found no similar operation in Aggregation class of Spring Aggregation Framework

推荐答案

更新:

从 Spring Data v2.0 开始，您可以这样做:

Starting with v2.0 of Spring Data you can do this:

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

<小时>

原答案:

像 spring-mongo 这样的抽象层总是会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.

Abstraction layers like spring-mongo are always going to lag way behind server released features. So you are best off contructing the BSON document structure for the pipeline stage yourself.

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:

And then use in your code:

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

由于它实现了 AggregationOperation，因此它可以很好地与现有的管道操作辅助方法一起使用.即:

Since this implements AggregationOperation this works well with the exising pipeline operation helper methods. i.e:

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")))
    )
);

再说一遍，归根结底，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.

So again, everything is just a BSON Object at the end of the day. It's just a matter of having an interface wrapper so that the class methods in spring-mongo interpret the result and get your defined BSON Object correctly.

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