JPA mapping tips

jpa-mapping-tips.txt

Start spring-boot app without depending on Database?
https://stackoverflow.com/questions/23850585/how-to-start-spring-boot-app-without-depending-on-database/23875516#23875516
Spring boot apps will start fine even if DB is down or there is no DB.
Apps will pick up the connections on the fly as DB comes up which means there is no need to restart the web server or redeploy the apps.
There is no need to start the tomcat or redeploy the apps, if DB goes down from running state and comes up again.
application.yml :

spring:
  datasource:
    driverClassName: com.mysql.jdbc.Driver
    url: jdbc:mysql://localhost:3306/schema
    username: root
    password: root
    continueOnError: true
    initialize: false
    initialSize: 0
    timeBetweenEvictionRunsMillis: 5000
    minEvictableIdleTimeMillis: 5000
    minIdle: 0

  jpa:
    show-sql: true
    hibernate:
      ddl-auto: none
      naming_strategy: org.hibernate.cfg.DefaultNamingStrategy
    properties:
      hibernate:   
        dialect: org.hibernate.dialect.MySQL5Dialect
        hbm2ddl:
          auto: none
        temp:
          use_jdbc_metadata_defaults: false

I had to do the same with EclipseLink, and all it took to do it was <property name="eclipselink.validation-only" value="true"/> in persistence.xml
====================================================
Don’t use unidirectional one-to-many associations
Avoid the mapping of huge to-many associations
better use an unidirectional many-to-one association
Think twice before using CascadeType.Remove
Use orphanRemoval when modeling parent-child associations
Implement helper methods to update bi-directional associations
Define FetchType.LAZY for @ManyToOne association

You can have a uni-directional mapping between Teams and Matches as follows:
In Matches Class:
@ManyToOne(optional=false)
@JoinColumn(name="home_team_id", referencedColumnName="team_id")
private Team homeTeam;

@ManyToOne(optional=false)
@JoinColumn(name="away_team_id" referencedColumnName="team_id")
private Team awayTeam;

If you need a bi-directional relationship you can add the following :
In Teams Class :
@OneToMany(mappedBy="homeTeam")
private Set<Matches> homeMatches;

@OneToMany(mappedBy="awayTeam")
private Set<Matches> awayMatches;

===========================
@Entity
public class Person implements Serializable {
    @Id Integer id;

    @OneToOne
    @JoinColumn(name = "id")
    Job myJob;
}
@Entity
public class Job implements Serializable {
    @Id Integer id;

    @OneToOne(mappedBy = "myJob")
    Person currentWorker;
}
(pay attemption to remove duplicated colum 'person_id' from Job)