How does JPA orphanRemoval true differ from the ON DELETE CASCADE DML clause

Question

I am a little confused about the JPA 2 0 orphanRemoval attribute   I think I can see it is needed when I use my JPA provider s DB generation tools to create the underlying database DDL to have an ON DELETE CASCADE on the particular relation   However  if the DB exists and it already has an ON DELETE CASCADE on the relation  is this not enough to cascade the deletion appropriately  What does the orphanRemoval do in addition   Cheers

User · Answer

The difference is   - orphanRemoval   true   Child  entity is removed when it s no longer referenced  its parent may not be removed    - CascadeType REMOVE   Child  entity is removed only when its  Parent  is removed

User · Answer

An example taken form here   When an Employee entity object is removed  the remove operation is cascaded to the referenced Address entity object  In this regard  orphanRemoval true and cascade CascadeType REMOVE are identical  and if orphanRemoval true is specified  CascadeType REMOVE is redundant   The difference between the two settings is in the response to disconnecting a relationship  For example  such as when setting the address field to null or to another Address object    If orphanRemoval true is specified the disconnected Address instance is automatically removed  This is useful for cleaning up dependent objects  e g  Address  that should not exist without a reference from an owner object  e g  Employee   If only cascade CascadeType REMOVE is specified  no automatic action is taken since disconnecting a relationship is not a remove operation    To avoid dangling references as a result of orphan removal  this feature should only be enabled for fields that hold private non shared dependent objects   I hope this makes it more clear

User · Answer

Entity state transitions JPA translates entity state transitions to SQL statements  like INSERT  UPDATE or DELETE   When you persist an entity  you are scheduling the INSERT statement to be executed when the EntityManager is flushed  either automatically or manually  when you remove an entity  you are scheduling the DELETE statement  which will be executed when the Persistence Context is flushed  Cascading entity state transitions For convenience  JPA allows you to propagate entity state transitions from parent entities to child one  So  if you have a parent Post entity that has a  OneToMany association with the PostComment child entity   The comments collection in the Post entity is mapped as follows   OneToMany      mappedBy    quot post quot        cascade   CascadeType ALL      orphanRemoval   true   private List lt Comment gt  comments   new ArrayList lt  gt      CascadeType ALL The cascade attribute tells the JPA provider to pass the entity state transition from the parent Post entity to all PostComment entities contained in the comments collection  So  if you remove the Post entity  Post post   entityManager find Post class  1L   assertEquals 2  post getComments   size      entityManager remove post    The JPA provider is going to remove the PostComment entity first  and when all child entities are deleted  it will delete the Post entity as well  DELETE FROM post comment WHERE id   1 DELETE FROM post comment WHERE id   2  DELETE FROM post WHERE id   1  Orphan removal When you set the orphanRemoval attribute to true  the JPA provider is going to schedule a remove operation when the child entity is removed from the collection  So  in our case  Post post   entityManager find Post class  1L   assertEquals 2  post getComments   size      PostComment postComment   post getComments   get 0   assertEquals 1L  postComment getId      post getComments   remove postComment    The JPA provider is going to remove the associated post comment record since the PostComment entity is no longer referenced in the comments collection  DELETE FROM post comment WHERE id   1  ON DELETE CASCADE The ON DELETE CASCADE is defined at the FK level  ALTER TABLE post comment  ADD CONSTRAINT fk post comment post id  FOREIGN KEY  post id  REFERENCES post  ON DELETE CASCADE   Once you do that  if you delete a post row  DELETE FROM post WHERE id   1  All the associated post comment entities are removed automatically by the database engine  However  this can be a very dangerous operation if you delete a root entity by mistake  Conclusion The advantage of the JPA cascade and orphanRemoval options is that you can also benefit from optimistic locking to prevent lost updates  If you use the JPA cascading mechanism  you don t need to use DDL-level ON DELETE CASCADE  which can be a very dangerous operation if you remove a root entity that has many child entities on multiple levels

User · Answer

The moment you remove a child entity from the collection you will also be removing that child entity from the DB as well  orphanRemoval also implies that you cannot change parents  if there s a department that has employees  once you remove that employee to put it in another deparment  you will have inadvertantly removed that employee from the DB at flush commit whichver comes first   The morale is to set orphanRemoval to true so long as you are certain that children of that parent will not migrate to a different parent throughout their existence  Turning on orphanRemoval also automatically adds REMOVE to cascade list

User · Answer

orphan removal has the same effect as ON DELETE CASCADE in the following scenario - Lets say we have a simple many to one relationship between student entity and a guide entity  where many students can be mapped to the same guide and in database we have a foreign key relation between Student and Guide table such that student table has id guide as FK        Entity      Table name    student   catalog    helloworld       public class Student implements java io Serializable         Id       GeneratedValue strategy   IDENTITY        Column name    id        private Integer id        ManyToOne cascade  CascadeType PERSIST CascadeType REMOVE        JoinColumn name    id guide       private Guide guide       The parent entity       Entity      Table name    guide   catalog    helloworld       public class Guide implements java io Serializable                private static final long serialVersionUID   9017118664546491038L    Id  GeneratedValue strategy   IDENTITY   Column name    id   unique   true  nullable   false  private Integer id    Column name    name   length   45  private String name    Column name    salary   length   45  private String salary      OneToMany mappedBy    guide   orphanRemoval true    private Set lt Student gt  students   new  HashSet lt Student gt  0     In this scenario  the relationship is such that student entity is the owner of the relationship and as such we need to save the student entity in order to persist the whole object graph e g       Guide guide   new Guide  John     1500        Student s1   new Student guide   Roy   ECE        Student s2   new Student guide   Nick    ECE        em persist s1       em persist s2     Here we are mapping the same guide with two different student objects and since the CASCADE PERSIST is used   the object graph will be saved as below in the database table MySql in my case   STUDENT table -  ID           Name          Dept    Id Guide  1   nbsp  nbsp  nbsp  nbsp Roy nbsp  nbsp  nbsp  nbsp  nbsp ECE nbsp  nbsp  nbsp  nbsp 1  2 nbsp  nbsp  nbsp  nbsp Nick nbsp  nbsp  nbsp  nbsp ECE nbsp  nbsp  nbsp  nbsp 1  GUIDE Table -  ID           NAME            Salary  1 nbsp  nbsp  nbsp  nbsp John nbsp  nbsp  nbsp  nbsp  nbsp  1500  and Now if I want to remove one of the students  using         Student student1   em find Student class 1         em remove student1     and when a student record is removed the corresponding guide record should also be removed  that s where CASCADE REMOVE attribute in the Student entity comes into picture and what it does is  it removes the student with identifier 1 as well the corresponding guide object identifier 1   But in this example  there is one more student object which is mapped to the same guide record and unless we use the orphanRemoval true attribute in the Guide Entity   the remove code above will not work

User · Answer

GaryK answer is absolutely great  I ve spent an hour looking for an explanation orphanRemoval   true vs CascadeType REMOVE and it helped me understand    Summing up  orphanRemoval   true works identical as CascadeType REMOVE ONLY IF we deleting object  entityManager delete object   and we want the childs objects to be removed as well   In completely different sitiuation  when we fetching some data like List lt Child gt  childs   object getChilds   and then remove a child  entityManager remove childs get 0   using orphanRemoval true will cause that entity corresponding to childs get 0  will be deleted from database

User · Answer

The equivalent JPA mapping for the DDL ON DELETE CASCADE is cascade CascadeType REMOVE  Orphan removal means that dependent entities are removed when the relationship to their  parent  entity is destroyed  For example if a child is removed from a  OneToMany relationship without explicitely removing it in the entity manager

User · Answer

orphanRemoval has nothing to do with ON DELETE CASCADE   orphanRemoval is an entirely ORM-specific thing  It marks  child  entity to be removed when it s no longer referenced from the  parent  entity  e g  when you remove the child entity from the corresponding collection of the parent entity   ON DELETE CASCADE is a database-specific thing  it deletes the  child  row in the database when the  parent  row is deleted

[hibernate] How does JPA orphanRemoval=true differ from the ON DELETE CASCADE DML clause

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