spring data操作mongodb中的聚合的例子

  先来看mongodb中的聚合操作：
有数据如下

{ "_id" : 1, "domainName" : "test1.com", "hosting" : "hostgator.com" }
{ "_id" : 2, "domainName" : "test2.com", "hosting" : "aws.amazon.com"}
{ "_id" : 3, "domainName" : "test3.com", "hosting" : "aws.amazon.com" }
{ "_id" : 4, "domainName" : "test4.com", "hosting" : "hostgator.com" }
{ "_id" : 5, "domainName" : "test5.com", "hosting" : "aws.amazon.com" }
{ "_id" : 6, "domainName" : "test6.com", "hosting" : "cloud.google.com" }
{ "_id" : 7, "domainName" : "test7.com", "hosting" : "aws.amazon.com" }
{ "_id" : 8, "domainName" : "test8.com", "hosting" : "hostgator.com" }
{ "_id" : 9, "domainName" : "test9.com", "hosting" : "cloud.google.com" }
{ "_id" : 10, "domainName" : "test10.com", "hosting" : "godaddy.com" }
...
{ "_id" : 100, "domainName" : "test10.com", "hosting" : "godaddy.com" }

  然后聚合操作为：
db.domain.aggregate(
    {
$match : {_id:{$lt:10}}
    },
    {
$group : {_id : "$hosting", total : { $sum : 1 }}
    },
    {
$sort : {total : -1}
    }
);

  输出：
{ "_id" : "aws.amazon.com", "total" : 4 }
{ "_id" : "hostgator.com", "total" : 3 }
{ "_id" : "cloud.google.com", "total" : 2 }
{ "_id" : "godaddy.com", "total" : 1 }

   使用spring data:

@Repository
public class DomainDaoImpl implements DomainDao {
 
	@Autowired
	MongoTemplate mongoTemplate;
 
	public List<HostingCount> getHostingCount() {
 
		Aggregation agg = newAggregation(
			match(Criteria.where("_id").lt(10)),
			group("hosting").count().as("total"),
			project("total").and("hosting").previousOperation(),
			sort(Sort.Direction.DESC, "total")
 
		);
 
		//Convert the aggregation result into a List
		AggregationResults<HostingCount> groupResults 
			= mongoTemplate.aggregate(agg, Domain.class, HostingCount.class);
		List<HostingCount> result = groupResults.getMappedResults();
 
		return result;
 
	}