MyBatis: How to map "inverse" relationship? - mybatis

Question

My problem is to persist two classes that have a 1:n relationship:
public class DayRecord {
private Long id;
private List<TimeRecord> timeRecordsToday = new ArrayList<TimeRecord>(4);
...
}
public class TimeRecord {
private Long id;
...
}
So, in code, DayRecord knows TimeRecord.
create table DAY_RECORDS (
id int primary key,
);
create table TIME_RECORDS (
id int primary key,
day_record_id int not null,
foreign key (day_record_id) references DAY_RECORDS (id)
);
In database, TimeRecord knows DayRecord.
Can I save a DayRecord with all its TimeRecords in one step?
In Hibernate, I can set an inverse mapping and just save a DayRecord and all its TimeRecords will get saved, too. With MyBatis, I tried to save the classes independently from each other:
<mapper
namespace="de.stevenschwenke.java.javafx.xyz.DayRecordMapper">
<insert id="insertDayRecord"
parameterType="de.stevenschwenke.java.javafx.xyz.DayRecord">
insert into DAY_RECORDS (id) values (NEXT VALUE FOR DAY_RECORDS_SEQ);
</insert>
</mapper>
<mapper
namespace="de.stevenschwenke.java.javafx.xyz.TimeRecordMapper">
<insert id="insertTimeRecord"
parameterType="de.stevenschwenke.java.javafx.xyz.TimeRecord">
insert into TIME_RECORDS (id) values (NEXT VALUE FOR TIME_RECORDS_SEQ);
</insert>
</mapper>
But how can I save the DayRecord-ID inTimeRecord?
Ideas:
Give TimeRecord an attribute dayRecordId. This way, a cyclic dependency would be created. However, the mapping would take care of the dpenedency while saving.
In one transaction, save the DayRecord first, get its ID, set it in TimeRecords and save this object.
use a nested select-statement within insert like in the documentation
What is the best way to save both objects? Thanks for your help!

Answer 1

As jdevelop already mentioned, MyBatis is just a SQL wrapper. Because SQL doesn't offer a way to insert two objects that have a relationship, MyBatis can't do that either.
So here's my workaround: As I mentioned, I don't want to add a circular dependency by letting TimeRecord know about DayRecord. So I created a wrapper class just for inserting TimeRecords:
public class TimeRecordInsertWrapper {
public Long id;
public int hours;
public long dayRecordId;
[constructor/getter/setter omited but there with public access modifier]
}
First, I store the DayRecord and get it's ID. Then I create the wrapper object and store the TimeRecords:
public long insertDayRecord(DayRecord newRecord) {
SqlSession session = sqlSessionFactory.openSession();
try {
session.insert(
"de.stevenschwenke.java.javafx.xyz.DayRecordMapper.insertDayRecord",
newRecord);
for (TimeRecord tr : newRecord.getTimeRecordsToday()) {
TimeRecordInsertWrapper wrapper = new TimeRecordInsertWrapper(tr.getHours(), newRecord.getId());
session.insert("de.stevenschwenke.java.javafx.xyz.TimeRecordMapper.insertTimeRecord",
wrapper);
}
return newRecord.getId();
} finally {
session.commit();
session.close();
}
}
This way, I can use my nice one-way object model AND have the "right" mapping in the database.

Answer 2

Mybatis is just SQL mapping framework, it allows you to abstract SQL code from Java code and that's it, more or less. They are pretending to look like Hibernate with recent versions, but this leads to weird constructions in XML.
I would suggest to store the DayRecord and get it's it from selectKey, then use that ID in subsequent calls to the mapper. This is what actually happens inside the mapper, but complex XML implies complex FSM to built inside. So keep it simple and you're safe with myBatis, or use Hibernate.
What is even better, you can define custom DAO interfaces for the tasks, and then you can have some sort of Service layer with #Transactional attribute set. This requires mybatis-guice, but it works really great and you don't need to deal with transactions in your code (they are declarative).