JPA difference between JoinColumn and PrimaryKeyJoinColumn

Question

What s the exact difference between  JoinColumn and  PrimaryKeyJoinColumn   You use  JoinColumn for columns that are part of a foreign key  A typical column could look like  e g  in a join table with additional attributes     ManyToOne  JoinColumn name          private OtherClass oc    What happens if I promote the column to be a the PK  too  a k a  identifying relationship   As the column is now the PK  I must tag it with  Id    Id  ManyToOne  JoinColumn name          private OtherClass oc    Now the question is   Are  Id    JoinColumn the same as just  PrimaryKeyJoinColumn     ManyToOne  PrimaryKeyJoinColumn name          private OtherClass oc    If not  what s  PrimaryKeyJoinColumn there for

User · Answer

I know this is an old post  but a good time to use PrimaryKeyColumn would be if you wanted a unidirectional relationship or had multiple tables all sharing the same id    In general this is a bad idea and it would be better to use foreign key relationships with JoinColumn    Having said that  if you are working on an older database that used a system like this then that would be a good time to use it

User · Answer

What happens if I promote the column to be a the PK  too  a k a  identifying relationship   As the column is now the PK  I must tag it with  Id          This enhanced support of derived identifiers is actually part of the new stuff in JPA 2 0  see the section 2 4 1 Primary Keys Corresponding to Derived Identities in the JPA 2 0 specification   JPA 1 0 doesn t allow Id on a OneToOne or ManyToOne  With JPA 1 0  you d have to use PrimaryKeyJoinColumn and also define a Basic Id mapping for the foreign key column      Now the question is  are  Id    JoinColumn the same as just  PrimaryKeyJoinColumn    You can obtain a similar result but using an Id on OneToOne or ManyToOne is much simpler and is the preferred way to map derived identifiers with JPA 2 0  PrimaryKeyJoinColumn might still be used in a JOINED inheritance strategy  Below the relevant section from the JPA 2 0 specification      11 1 40 PrimaryKeyJoinColumn Annotation      The PrimaryKeyJoinColumn annotation   specifies a primary key column that is   used as a foreign key to join to   another table       The PrimaryKeyJoinColumn annotation   is used to join the primary table of   an entity subclass in the JOINED   mapping strategy to the primary table   of its superclass  it is used within a   SecondaryTable annotation to join a   secondary table to a primary table    and it may be used in a OneToOne   mapping in which the primary key of   the referencing entity is used as a   foreign key to the referenced   entity 108                 If no PrimaryKeyJoinColumn   annotation is specified for a subclass   in the JOINED mapping strategy  the   foreign key columns are assumed to   have the same names as the primary key   columns of the primary table of the   superclass                Example  Customer and ValuedCustomer subclass   Entity  Table name  CUST    Inheritance strategy JOINED   DiscriminatorValue  CUST   public class Customer           Entity  Table name  VCUST    DiscriminatorValue  VCUST    PrimaryKeyJoinColumn name  CUST ID   public class ValuedCustomer extends Customer                108  The derived id mechanisms   described in section 2 4 1 1 are now   to be preferred over   PrimaryKeyJoinColumn for the   OneToOne mapping case    See also   Primary Keys through OneToOne Relationships        This source http   weblogs java net blog felipegaucho archive 2009 10 24 jpa-join-table-additional-state states that using  ManyToOne and  Id works with JPA 1 x  Who s correct now    The author is using a pre release JPA 2 0 compliant version of EclipseLink  version 2 0 0-M7 at the time of the article  to write an article about JPA 1 0     This article is misleading  the author is using something that is NOT part of JPA 1 0   For the record  support of Id on OneToOne and ManyToOne has been added in EclipseLink 1 1  see this message from James Sutherland  EclipseLink comitter and main contributor of the Java Persistence wiki book   But let me insist  this is NOT part of JPA 1 0

User · Answer

I normally differentiate these two via this diagram   Use PrimaryKeyJoinColumn    Use JoinColumn

[java] JPA: difference between @JoinColumn and @PrimaryKeyJoinColumn?

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