聚合 和 分组聚合：

PlayMorphia 给开发者提供了基于models的组合和分组的友好接口

设想你定义了一个model，class Sales: 

@Entity public class Sales extends Model {
    public String employeeId;
    public String department;
    public String region;
    public int amount;
}

long total = Sales.count(); // also Sales.q().count();
long cntIT = Sales.q().filter("department", "IT").count();
long cntAU = Sales.q().filter("region", "AU").count();
long cntAuIt = Sales.find("region, department", "AU", "IT").count();

long sum = Sales._sum("amount"); // also Sales.q().sum("amount");
long sumIT = Sales.find("department", "IT").sum("amount");
long sumAU = Sales.find("region", "AU").sum("amount");
long sumAuIt = Sales.find("region department", "AU", "IT").sum("amount");

long max = Sales._max("amount"); // also Sales.q().max("amount");
long maxIT = Sales.find("department", "IT").max("amount");
long maxAU = Sales.find("region", "AU").max("amount");
long maxAuIt = Sales.find("region department", "AU", "IT").max("amount");

long min = Sales._min("amount"); // also Sales.q().min("amount");
long minIT = Sales.find("department", "IT").min("amount");
long minAU = Sales.find("region", "AU").min("amount");
long minAuIt = Sales.find("region department", "AU", "IT").min("amount");

每一个聚合都会对应一个分组聚合的接口，就像SQL中的group by语句

分组计数：

// group by region
AggregationResult byRegion = Sales.groupCount("region");
System.out.println("AU count: " + byRegion.get("region", "AU");
// group by department
AggregationResult byDep = Sales.groupCount("department");
System.out.println("IT count: " + byDep.get("department", "IT");
// group by region and department
AggregationResult byRegionDep = Sales.groupCount("region, department");
System.out.println("IT count: " + byRegionDep.get("department, region", "IT", "AU");

// group by region
AggregationResult byRegion = Sales.groupSum("region");
System.out.println("AU sum: " + byRegion.get("region", "AU");
// group by department
AggregationResult byDep = Sales.groupSum("department");
System.out.println("IT sum: " + byDep.get("department", "IT");
// group by region and department
AggregationResult byRegionDep = Sales.groupSum("region, department");
System.out.println("IT sum: " + byRegionDep.get("department, region", "IT", "AU");

// group by region
AggregationResult byRegion = Sales.groupMax("region");
System.out.println("AU max: " + byRegion.get("region", "AU");
// group by department
AggregationResult byDep = Sales.groupMax("department");
System.out.println("IT max: " + byDep.get("department", "IT");
// group by region and department
AggregationResult byRegionDep = Sales.groupMax("region, department");
System.out.println("IT max: " + byRegionDep.get("department, region", "IT", "AU");

// group by region
AggregationResult byRegion = Sales.groupMin("region");
System.out.println("AU min: " + byRegion.get("region", "AU");
// group by department
AggregationResult byDep = Sales.groupMin("department");
System.out.println("IT min: " + byDep.get("department", "IT");
// group by region and department
AggregationResult byRegionDep = Sales.groupMin("region, department");
System.out.println("IT min: " + byRegionDep.get("department, region", "IT", "AU");