In which case do you use the JPA JoinTable annotation

Question

In which case do you use the JPA  JoinTable annotation

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It lets you handle Many to Many relationship  Example   Table 1  post  post has following columns                         ID        DATE                                                                        Table 2  user  user has the following columns                              ID   NAME                                                                           Join Table lets you create a mapping using    JoinTable    name  USER POST     joinColumns  JoinColumn name  USER ID   referencedColumnName  ID      inverseJoinColumns  JoinColumn name  POST ID   referencedColumnName  ID      will create a table                           USER ID  POST ID

User · Answer

It s the only solution to map a ManyToMany association   you need a join table between the two entities tables to map the association    It s also used for OneToMany  usually unidirectional  associations when you don t want to add a foreign key in the table of the many side and thus keep it independent of the one side    Search for  JoinTable in the hibernate documentation for explanations and examples

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EDIT 2017-04-29  As pointed to by some of the commenters  the JoinTable example does not need the mappedBy annotation attribute  In fact  recent versions of Hibernate refuse to start up by printing the following error   org hibernate AnnotationException      Associations marked as mappedBy must not define database mappings     like  JoinTable or  JoinColumn     Let s pretend that you have an entity named Project and another entity named Task and each project can have many tasks   You can design the database schema for this scenario in two ways   The first solution is to create a table named Project and another table named Task and add a foreign key column to the task table named project id    Project      Task -------      ---- id           id name         name              project id   This way  it will be possible to determine the project for each row in the task table  If you use this approach  in your entity classes you won t need a join table    Entity public class Project        OneToMany mappedBy    project      private Collection lt Task gt  tasks       Entity public class Task        ManyToOne    private Project project       The other solution is to use a third table  e g  Project Tasks  and store the relationship between projects and tasks in that table   Project      Task      Project Tasks -------      ----      ------------- id           id        project id name         name      task id   The Project Tasks table is called a  Join Table   To implement this second solution in JPA you need to use the  JoinTable annotation  For example  in order to implement a uni-directional one-to-many association  we can define our entities as such   Project entity    Entity public class Project         Id      GeneratedValue     private Long pid       private String name        JoinTable      OneToMany     private List lt Task gt  tasks       public Long getPid             return pid             public void setPid Long pid            this pid   pid             public String getName             return name             public void setName String name            this name   name             public List lt Task gt  getTasks             return tasks             public void setTasks List lt Task gt  tasks            this tasks   tasks            Task entity    Entity public class Task         Id      GeneratedValue     private Long tid       private String name       public Long getTid             return tid             public void setTid Long tid            this tid   tid             public String getName             return name             public void setName String name            this name   name             This will create the following database structure     The  JoinTable annotation also lets you customize various aspects of the join table  For example  had we annotated the tasks property like this    JoinTable          name    MY JT           joinColumns    JoinColumn                  name    PROJ ID                   referencedColumnName    PID                     inverseJoinColumns    JoinColumn                  name    TASK ID                   referencedColumnName    TID               OneToMany private List lt Task gt  tasks    The resulting database would have become     Finally  if you want to create a schema for a many-to-many association  using a join table is the only available solution

User · Answer

ManyToMany associations Most often  you will need to use  JoinTable annotation to specify the mapping of a many-to-many table relationship   the name of the link table and the two Foreign Key columns  So  assuming you have the following database tables   In the Post entity  you would map this relationship  like this   ManyToMany cascade         CascadeType PERSIST      CascadeType MERGE     JoinTable      name    quot post tag quot       joinColumns    JoinColumn name    quot post id quot        inverseJoinColumns    JoinColumn name    quot tag id quot     private List lt Tag gt  tags   new ArrayList lt  gt      The  JoinTable annotation is used to specify the table name via the name attribute  as well as the Foreign Key column that references the post table  e g   joinColumns  and the Foreign Key column in the post tag link table that references the Tag entity via the inverseJoinColumns attribute   Notice that the cascade attribute of the  ManyToMany annotation is set to PERSIST and MERGE only because cascading REMOVE is a bad idea since we the DELETE statement will be issued for the other parent record  tag in our case  not to the post tag record   Unidirectional  OneToMany associations The unidirectional  OneToMany associations  that lack a  JoinColumn mapping  behave like many-to-many table relationships  rather than one-to-many  So  assuming you have the following entity mappings   Entity name    quot Post quot    Table name    quot post quot   public class Post          Id      GeneratedValue     private Long id        private String title         OneToMany          cascade   CascadeType ALL          orphanRemoval   true           private List lt PostComment gt  comments   new ArrayList lt  gt             Constructors  getters and setters removed for brevity      Entity name    quot PostComment quot    Table name    quot post comment quot   public class PostComment          Id      GeneratedValue     private Long id        private String review          Constructors  getters and setters removed for brevity    Hibernate will assume the following database schema for the above entity mapping   As already explained  the unidirectional  OneToMany JPA mapping behaves like a many-to-many association  To customize the link table  you can also use the  JoinTable annotation   OneToMany      cascade   CascadeType ALL      orphanRemoval   true    JoinTable      name    quot post comment ref quot       joinColumns    JoinColumn name    quot post id quot        inverseJoinColumns    JoinColumn name    quot post comment id quot     private List lt PostComment gt  comments   new ArrayList lt  gt      And now  the link table is going to be called post comment ref and the Foreign Key columns will be post id  for the post table  and post comment id  for the post comment table   Unidirectional  OneToMany associations are not efficient  so you are better off using bidirectional  OneToMany associations or just the  ManyToOne side

User · Answer

It s also cleaner to use  JoinTable when an Entity could be the child in several parent child relationships with different types of parents   To follow up with Behrang s example  imagine a Task can be the child of Project  Person  Department  Study  and Process   Should the task table have 5 nullable foreign key fields   I think not

[java] In which case do you use the JPA @JoinTable annotation?

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