在Spring data mongo DB中，我们如何获取城市及其级别计数

@Document(collection = "cities")
public class City {
    @Id
    private String id;
    private String name;
    private String level;
    
    // getters and setters
}

Aggregation aggregation = Aggregation.newAggregation(
    Aggregation.group("name", "level").count().as("count"),
    Aggregation.project("name", "level", "count")
);

AggregationResults<CityCount> results = mongoTemplate.aggregate(aggregation, "cities", CityCount.class);
List<CityCount> cityCounts = results.getMappedResults();

public class CityCount {
    private String name;
    private String level;
    private int count;
    
    // getters and setters
}

for (CityCount cityCount : cityCounts) {
    System.out.println("城市：" + cityCount.getName());
    System.out.println("级别：" + cityCount.getLevel());
    System.out.println("计数：" + cityCount.getCount());
}