What s the difference between JoinColumn and mappedBy when using a JPA OneToMany association

Question

What is the difference between    Entity public class Company         OneToMany cascade   CascadeType ALL   fetch   FetchType LAZY       JoinColumn name    companyIdRef   referencedColumnName    companyId       private List lt Branch gt  branches              and    Entity public class Company         OneToMany cascade   CascadeType ALL   fetch   FetchType LAZY  mappedBy    companyIdRef       private List lt Branch gt  branches

User · Answer

JPA is a layered API  the different levels have their own annotations  The highest level is the  1  Entity level which describes persistent classes then you have the  2  relational database level which assume the entities are mapped to a relational database and  3  the java model   Level 1 annotations   Entity   Id   OneToOne   OneToMany   ManyToOne   ManyToMany  You can introduce persistency in your application using these high level annotations alone  But then you have to create your database according to the assumptions JPA makes  These annotations specify the entity relationship model   Level 2 annotations   Table   Column   JoinColumn      Influence the mapping from entities properties to the relational database tables columns if you are not satisfied with JPA s defaults or if you need to map to an existing database  These annotations can be seen as implementation annotations  they specify how the mapping should be done   In my opinion it is best to stick as much as possible to the high level annotations and then introduce the lower level annotations as needed   To answer the questions  the  OneToMany mappedBy is nicest because it only uses the annotations from the entity domain  The  oneToMany  JoinColumn is also fine but it uses an implementation annotation where this is not strictly necessary

User · Answer

Let me make it simple   You can use  JoinColumn on either sides irrespective of mapping    Let s divide this into three cases  1  Uni-directional mapping from Branch to Company  2  Bi-direction mapping from Company to Branch  3  Only Uni-directional mapping from Company to Branch   So any use-case will fall under this three categories  So let me explain how to use  JoinColumn and mappedBy  1  Uni-directional mapping from Branch to Company  Use JoinColumn in Branch table  2  Bi-direction mapping from Company to Branch  Use mappedBy in Company table as describe by  Mykhaylo Adamovych s answer  3 Uni-directional mapping from Company to Branch  Just use  JoinColumn in Company table    Entity public class Company     OneToMany cascade   CascadeType ALL   fetch   FetchType LAZY   JoinColumn name  courseId   private List lt Branch gt  branches          This says that in based on the foreign key  courseId  mapping in branches table  get me list of all branches  NOTE  you can t fetch company from branch in this case  only uni-directional mapping exist from company to branch

User · Answer

JoinColumn could be used on both sides of the relationship  The question was about using  JoinColumn on the  OneToMany side  rare case   And the point here is in physical information duplication  column name  along with not optimized SQL query that will produce some additional UPDATE statements   According to documentation   Since many to one are  almost  always the owner side of a bidirectional relationship in the JPA spec  the one to many association is annotated by  OneToMany mappedBy        Entity public class Troop        OneToMany mappedBy  troop       public Set lt Soldier gt  getSoldiers                 Entity public class Soldier        ManyToOne      JoinColumn name  troop fk       public Troop getTroop                  Troop has a bidirectional one to many relationship with Soldier through the troop property  You don t have to  must not  define any physical mapping in the mappedBy side   To map a bidirectional one to many  with the one-to-many side as the owning side  you have to remove the mappedBy element and set the many to one  JoinColumn as insertable and updatable to false  This solution is not optimized and will produce some additional UPDATE statements    Entity public class Troop        OneToMany      JoinColumn name  troop fk     we need to duplicate the physical information     public Set lt Soldier gt  getSoldiers                 Entity public class Soldier        ManyToOne      JoinColumn name  troop fk   insertable false  updatable false      public Troop getTroop

User · Answer

I disagree with the accepted answer here by   scar L  pez  That answer is inaccurate     It is NOT  JoinColumn which indicates that this entity is the owner of the relationship   Instead  it is the  ManyToOne annotation which does this  in his example    The relationship annotations such as  ManyToOne   OneToMany and  ManyToMany tell JPA Hibernate to create a mapping   By default  this is done through a seperate Join Table     JoinColumn      The purpose of  JoinColumn is to create a join column if one does   not already exist   If it does  then this annotation can be used to   name the join column     MappedBy      The purpose of the MappedBy parameter is to instruct JPA  Do NOT   create another join table as the relationship is already being mapped   by the opposite entity of this relationship        Remember   MappedBy is a property of the relationship annotations whose purpose is to generate a mechanism to relate two entities which by default they do by creating a join table   MappedBy halts that process in one direction   The entity not using MappedBy is said to be the owner of the relationship because the mechanics of the mapping are dictated within its class through the use of one of the three mapping annotations against the foreign key field   This not only specifies the nature of the mapping but also instructs the creation of a join table   Furthermore  the option to suppress the join table also exists by applying  JoinColumn annotation over the foreign key which keeps it inside the table of the owner entity instead   So in summary   JoinColumn either creates a new join column or renames an existing one  whilst the MappedBy parameter works collaboratively with the relationship annotations of the other  child  class in order to create a mapping either through a join table or by creating a foreign key column in the associated table of the owner entity   To illustrate how MapppedBy works  consider the code below   If MappedBy parameter were to be deleted  then Hibernate would actually create TWO join tables   Why   Because there is a symmetry in many-to-many relationships and Hibernate has no rationale for selecting one direction over the other   We therefore use MappedBy to tell Hibernate  we have chosen the other entity to dictate the mapping of the relationship between the two entities    Entity public class Driver        ManyToMany mappedBy    drivers       private List lt Cars gt  cars      Entity public class Cars        ManyToMany     private List lt Drivers gt  drivers      Adding  JoinColumn name    driverID   in the owner class  see below   will prevent the creation of a join table and instead  create a driverID foreign key column in the Cars table to construct a mapping    Entity public class Driver        ManyToMany mappedBy    drivers       private List lt Cars gt  cars      Entity public class Cars        ManyToMany      JoinColumn name    driverID       private List lt Drivers gt  drivers

User · Answer

The annotation mappedBy ideally should always be used in the Parent side  Company class  of the bi directional relationship  in this case it should be in Company class pointing to the member variable  company  of the Child class  Branch class    The annotation  JoinColumn is used to specify a mapped column for joining an entity association  this annotation can be used in any class  Parent or Child  but it should ideally be used only in one side  either in parent class or in Child class not in both  here in this case i used it in the Child side  Branch class  of the bi directional relationship indicating the foreign key in the Branch class   below is the working example    parent class   Company   Entity public class Company         private int companyId      private String companyName      private List lt Branch gt  branches        Id      GeneratedValue      Column name  COMPANY ID       public int getCompanyId             return companyId             public void setCompanyId int companyId            this companyId   companyId              Column name  COMPANY NAME       public String getCompanyName             return companyName             public void setCompanyName String companyName            this companyName   companyName              OneToMany fetch FetchType LAZY cascade CascadeType ALL mappedBy  company       public List lt Branch gt  getBranches             return branches             public void setBranches List lt Branch gt  branches            this branches   branches              child class  Branch   Entity public class Branch        private int branchId      private String branchName      private Company company        Id      GeneratedValue      Column name  BRANCH ID       public int getBranchId             return branchId             public void setBranchId int branchId            this branchId   branchId              Column name  BRANCH NAME       public String getBranchName             return branchName             public void setBranchName String branchName            this branchName   branchName              ManyToOne fetch FetchType LAZY       JoinColumn name  COMPANY ID       public Company getCompany             return company             public void setCompany Company company            this company   company

User · Answer

Unidirectional one-to-many association If you use the  OneToMany annotation with  JoinColumn  then you have a unidirectional association  like the one between the parent Post entity and the child PostComment in the following diagram   When using a unidirectional one-to-many association  only the parent side maps the association  In this example  only the Post entity will define a  OneToMany association to the child PostComment entity   OneToMany cascade   CascadeType ALL  orphanRemoval   true   JoinColumn name    quot post id quot   private List lt PostComment gt  comments   new ArrayList lt  gt      Bidirectional one-to-many association If you use the  OneToMany with the mappedBy attribute set  you have a bidirectional association  In our case  both the Post entity has a collection of PostComment child entities  and the child PostComment entity has a reference back to the parent Post entity  as illustrated by the following diagram   In the PostComment entity  the post entity property is mapped as follows   ManyToOne fetch   FetchType LAZY  private Post post    The reason we explicitly set the fetch attribute to FetchType LAZY is because  by default  all  ManyToOne and  OneToOne associations are fetched eagerly  which can cause N 1 query issues   In the Post entity  the comments association is mapped as follows   OneToMany      mappedBy    quot post quot       cascade   CascadeType ALL      orphanRemoval   true   private List lt PostComment gt  comments   new ArrayList lt  gt      The mappedBy attribute of the  OneToMany annotation references the post property in the child PostComment entity  and  this way  Hibernate knows that the bidirectional association is controlled by the  ManyToOne side  which is in charge of managing the Foreign Key column value this table relationship is based on  For a bidirectional association  you also need to have two utility methods  like addChild and removeChild  public void addComment PostComment comment        comments add comment       comment setPost this      public void removeComment PostComment comment        comments remove comment       comment setPost null      These two methods ensure that both sides of the bidirectional association are in sync  Without synchronizing both ends  Hibernate does not guarantee that association state changes will propagate to the database  Which one to choose  The unidirectional  OneToMany association does not perform very well  so you should avoid it  You are better off using the bidirectional  OneToMany which is more efficient

User · Answer

The annotation  JoinColumn indicates that this entity is the owner of the relationship  that is  the corresponding table has a column with a foreign key to the referenced table   whereas the attribute mappedBy indicates that the entity in this side is the inverse of the relationship  and the owner resides in the  quot other quot  entity  This also means that you can access the other table from the class which you ve annotated with  quot mappedBy quot   fully bidirectional relationship   In particular  for the code in the question the correct annotations would look like this   Entity public class Company        OneToMany mappedBy    quot company quot                  orphanRemoval   true                 fetch   FetchType LAZY                 cascade   CascadeType ALL      private List lt Branch gt  branches      Entity public class Branch        ManyToOne fetch   FetchType LAZY       JoinColumn name    quot companyId quot       private Company company

User · Answer

I d just like to add that  JoinColumn does not always have to be related to the physical information location as this answer suggests   You can combine  JoinColumn with  OneToMany even if the parent table has no table data pointing to the child table   How to define unidirectional OneToMany relationship in JPA  Unidirectional OneToMany  No Inverse ManyToOne  No Join Table  It seems to only be available in JPA 2 x  though   It s useful for situations where you want the child class to just contain the ID of the parent  not a full on reference

[java] What's the difference between @JoinColumn and mappedBy when using a JPA @OneToMany association

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