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 {

    @Id
    private 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