例如当前test库中的grade集合中有两条文档数据，如下图所示：

相关的两个实体映射类如下： 

import lombok.Data;
import org.springframework.data.annotation.Id;
import org.springframework.data.mongodb.core.mapping.Document;
import org.springframework.data.mongodb.core.mapping.Field;import java.util.List;@Data
@Document(value = "grade")
public class Grade {@Idprivate Integer id;@Field(value = "grade_name")private String gradeName;@Field(value = "students")private List<Student> students;
}

import lombok.Data;
import org.springframework.data.mongodb.core.mapping.Field;@Data
public class Student {@Field(value = "student_id")private int studentId;@Field(value