spring-data-mongo 实现aggregation

摘要

mongo aggregation是mongo 的一个轻量级的map-reduce框架，可以实现一些count,sum,groupby的聚合。本身的意义就在于减少查询次数，将原本需要客户端的多次查询能够整合到db端的基于pipeline的查询。这样带来的好处就是减少请求数据库次数，坏处就是这个查询占用的数据库内存大，时间查，有可能拖垮其他查询。这个在本篇文章不讨论。

概述

常用的阶段函数

• $match： 就是where查询

• $group : 就是group by

• $project：添加新字段/重置已有字段，比如group阶段的key值会变成_id就需要

• $unwind :将数组拆分为几个字段

• $sort 排序

• $limit 限制条数

• $skip 跳过条数

例子

1. java 配置 mongo 连接

   @Configuration
   @EnableMongoRepositories(basePackages = "com.fs.mongo.dao")
   public class MongoConfig extends AbstractMongoConfiguration {
   
       @Bean
       public MongoDbFactory mongoDbFactory() throws UnknownHostException {
           MongoClientOptions options = new MongoClientOptions.Builder().connectionsPerHost(8).build();
           return new SimpleMongoDbFactory(new MongoClient("localhost",options ),"test");
       }
       // 默认数据库会生成_class字段，需要更改mappingMongoConverter的typeMappper属性值
       @Bean
       public MongoTemplate mongoTemplate() throws Exception {
           return new MongoTemplate(mongoDbFactory(),mappingMongoConverter());
       }
   
       protected String getDatabaseName() {
           return "test";
       }
   
       @Bean
       public Mongo mongo() throws Exception {
           return new MongoClient();
       }
   
       @Bean
       @Override
       public MappingMongoConverter mappingMongoConverter() throws Exception {
           MappingMongoConverter converter = new         MappingMongoConverter(mongoDbFactory(),this.mongoMappingContext());
           converter.setTypeMapper(new DefaultMongoTypeMapper(null));
           return converter;
       }
   }

2. 基于mongoTemplate 发送aggregate请求

   @Component
   public class PostDb {
       @Autowired
       private MongoTemplate mongoTemplate;
   
       public PostDb(MongoTemplate mongoTemplate) {
           this.mongoTemplate = mongoTemplate;
       }
       public List<Post> getPeopleAggregationByCategory(long userId) {
           MatchOperation match = Aggregation.match(new Criteria("userId").is(userId));
           GroupOperation group  = Aggregation.group("category").count().as("total");
           // 注group key category会映射成_id，所以要利用project阶段映射回category
           ProjectionOperation project =  Aggregation.project("total").and("_id").as("category");
           Aggregation aggregation = Aggregation.newAggregation(match,group,project);
           AggregationResults<Post> results = mongoTemplate.aggregate(aggregation,"post",Post.class);
           return results.getMappedResults();
       }
   }

完整代码：
https://github.com/FS1360472174/nosql-mongo/tree/master/project/sample/sample-mongo-aggregation

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