What is referencedColumnName used for in JPA

Question

In JPA there is an attribute called referencedColumnName that can be set on  JoinColumn   PrimaryKeyJoinColumn what is the idea behind this setting  can someone give a good example of where this can be used

User · Answer

Quoting API on referencedColumnName      The name of the column referenced by this foreign key   column       Default  only applies if single join column is being used     The same name as the primary key column of the referenced table    Q A     Where this would be used    When there is a composite PK in referenced table  then you need to specify column name you are referencing

User · Answer

It is there to specify another column as the default id column of the other table  e g  consider the following  TableA   id int identity   tableb key varchar   TableB   id int identity   key varchar unique     in class for TableA  JoinColumn name  tableb key   referencedColumnName  key

User · Answer

referencedColumnName  property is the name of the column in the table that you are making reference with the column you are anotating  Or in a short manner  it s the column referenced in the destination table  Imagine something like this  cars and persons  One person can have many cars but one car belongs only to one person  sorry  I don t like anyone else driving my car        Table Person    name char 64  primary key   age int      Table Car    car registration char 32  primary key   car brand  char 64    car model  char64    owner name char 64  foreign key references Person name    When you implement classes you will have something like  class Person            class Car               ManyToOne      JoinColumn  column name  owner name   referencedColumnName  name       private Person owner      EDIT  as  searchengine27 has commented  columnName does not exist as a field in persistence section of Java7 docs  I can t remember where I took this property from  but I remember using it  that s why I m leaving it in my example

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name attribute points to the column containing the asociation  i e  column name of the foreign key referencedColumnName attribute points to the related column in asociated referenced entity  i e  column name of the primary key   You are not required to fill the referencedColumnName if the referenced entity has single column as PK  because there is no doubt what column it references  i e  the Address single column ID     ManyToOne  JoinColumn name  ADDR ID   public Address getAddress     return address      However if the referenced entity has PK that spans multiple columns the order in which you specify  JoinColumn annotations has significance  It might work without the referencedColumnName specified  but that is just by luck  So you should map it like this    ManyToOne  JoinColumns        JoinColumn name  ADDR ID   referencedColumnName  ID         JoinColumn name  ADDR ZIP   referencedColumnName  ZIP      public Address getAddress     return address      or in case of ManyToMany    ManyToMany  JoinTable      name  CUST ADDR       joinColumns           JoinColumn name  CUST ID        inverseJoinColumns            JoinColumn name  ADDR ID   referencedColumnName  ID             JoinColumn name  ADDR ZIP   referencedColumnName  ZIP             Real life example  Two queries generated by Hibernate of the same join table mapping  both without referenced column specified  Only the order of  JoinColumn annotations were changed      load collection Client emails     select  emails0  id client as id1 18 1   emails0  rev as rev18 1   emails0  id email as id3 1   email1  id email as id1 6 0   from client email emails0   inner join email email1  on emails0  id email email1  id email   where emails0  id client  2  and  emails0  rev  18       load collection Client emails     select emails0  rev as rev18 1   emails0  id client as id2 18 1   emails0  id email as id3 1    email1  id email as id1 6 0   from client email emails0   inner join email email1  on emails0  id email email1  id email   where emails0  rev  2  and  emails0  id client  18    We are querying a join table to get client s emails  The  2  18  is composite ID of Client  The order of column names is determined by your order of  JoinColumn annotations  The order of both integers is always the same  probably sorted by hibernate and that s why proper alignment with join table columns is required and we can t or should rely on mapping order   The interesting thing is the order of the integers does not match the order in which they are mapped in the entity - in that case I would expect  18  2   So it seems the Hibernate is sorting the column names before it use them in query  If this is true and you would order your  JoinColumn in the same way you would not need referencedColumnName  but I say this only for illustration   Properly filled referencedColumnName attributes result in exactly same query without the ambiguity  in my case the second query  rev   2  id client   18

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For a JPA 2 x example usage for the general case of two tables  with a  OneToMany unidirectional join see https   en wikibooks org wiki Java Persistence OneToMany Example of a JPA 2 x unidirectional OneToMany relationship annotations  Screenshot from this WikiBooks JPA article  Example of a JPA 2 x unidirectional OneToMany relationship database

[hibernate] What is referencedColumnName used for in JPA?

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