Mybatis One-to-many mapping

We can map one-to-many types of results to a collection of objects using the element.The JavaBeans for Course and Tutor are as follows:

public class Course
{
 private Integer courseId;
 private String name;
 private String description;
 private Date startDate;
 private Date endDate;
 private Integer tutorId;

 //setters & getters
}
public class Tutor
{
 private Integer tutorId;
 private String name;
 private String email;
 private Address address;
 private List courses;
 //setters & getters
}

The element can be used to map multiple course rows to a list of course objects. Similar to one-to-one mapping, we can map one-to-many relationships using a nested ResultMap and nested Select approaches.

1. One-to-many mapping with nested ResultMap
We can get the tutor along with the courses' details using a nested ResultMap as follows:

 
 
 
 
 
 


 
 
 
 



 SELECT T.TUTOR_ID, T.NAME AS TUTOR_NAME, EMAIL, C.COURSE_ID,
 C.NAME, DESCRIPTION, START_DATE, END_DATE
 FROM TUTORS T LEFT OUTER JOIN ADDRESSES A ON T.ADDR_ID=A.ADDR_ID
 LEFT OUTER JOIN COURSES C ON T.TUTOR_ID=C.TUTOR_ID
 WHERE T.TUTOR_ID=#{tutorId}

Here we are fetching the tutor along with the courses' details using a single Select query with JOINS. The element's resultMap is set to the resultMap ID CourseResult that contains the mapping for the Course object's properties.

2. One-to-many mapping with nested select
We can get the tutor along with the courses' details using a nested select query as follows:

 
 
 
 
 



 
 
 
 
 
 

 
 SELECT T.TUTOR_ID, T.NAME AS TUTOR_NAME, EMAIL
 FROM TUTORS T WHERE T.TUTOR_ID=#{tutorId}
 

 SELECT * FROM COURSES WHERE TUTOR_ID=#{tutorId}

In this approach, the element's select attribute is set to the statement ID findCoursesByTutor that triggers a separate SQL query to load the courses' details. The tutor_id column value will be passed as input to the findCoursesByTutor statement.