Can someone explain mappedBy in JPA and Hibernate

Question

I am new to hibernate and need to use one-to-many and many-to-one relations  It is a bi-directional relationship in my objects  so that I can traverse from either direction  mappedBy is the recommended way to go about it  however  I couldn t understand it  Can someone explain    what is the recommended way to use it  what purpose does it solve    For the sake of my example  here are my classes with annotations    Airline OWNS many AirlineFlights Many AirlineFlights belong to ONE Airline   Airline    Entity   Table name  Airline   public class Airline       private Integer idAirline      private String name       private String code       private String aliasName      private Set lt AirlineFlight gt  airlineFlights   new HashSet lt AirlineFlight gt  0        public Airline          public Airline String name  String code  String aliasName  Set lt AirlineFlight gt  flights            setName name           setCode code           setAliasName aliasName           setAirlineFlights flights               Id      GeneratedValue strategy GenerationType IDENTITY       Column name  IDAIRLINE   nullable false      public Integer getIdAirline             return idAirline             private void setIdAirline Integer idAirline            this idAirline   idAirline              Column name  NAME   nullable false      public String getName             return name            public void setName String name            this name   DAOUtil convertToDBString name               Column name  CODE   nullable false  length 3      public String getCode             return code            public void setCode String code            this code   DAOUtil convertToDBString code               Column name  ALIAS   nullable true      public String getAliasName             return aliasName            public void setAliasName String aliasName            if aliasName    null              this aliasName   DAOUtil convertToDBString aliasName               OneToMany fetch FetchType LAZY  cascade    CascadeType ALL        JoinColumn name  IDAIRLINE       public Set lt AirlineFlight gt  getAirlineFlights             return airlineFlights             public void setAirlineFlights Set lt AirlineFlight gt  flights            this airlineFlights   flights            AirlineFlights    Entity  Table name  AirlineFlight   public class AirlineFlight       private Integer idAirlineFlight      private Airline airline      private String flightNumber       public AirlineFlight          public AirlineFlight Airline airline  String flightNumber            setAirline airline           setFlightNumber flightNumber               Id      GeneratedValue generator  identity        GenericGenerator name  identity   strategy  identity        Column name  IDAIRLINEFLIGHT   nullable false      public Integer getIdAirlineFlight             return idAirlineFlight            private void setIdAirlineFlight Integer idAirlineFlight            this idAirlineFlight   idAirlineFlight              ManyToOne fetch FetchType LAZY       JoinColumn name  IDAIRLINE   nullable false      public Airline getAirline             return airline            public void setAirline Airline airline            this airline   airline              Column name  FLIGHTNUMBER   nullable false      public String getFlightNumber             return flightNumber            public void setFlightNumber String flightNumber            this flightNumber   DAOUtil convertToDBString flightNumber             EDIT   Database schema   AirlineFlights has the idAirline as ForeignKey and Airline has no idAirlineFlights  This makes  AirlineFlights as the owner identifying entity    Theoretically  I would like airline to be the owner of airlineFlights

User · Answer

Table relationship vs  entity relationship In a relational database system  a one-to-many table relationship looks as follows   Note that the relationship is based on the Foreign Key column  e g   post id  in the child table  So  there is a single source of truth when it comes to managing a one-to-many table relationship  Now  if you take a bidirectional entity relationship that maps on the one-to-many table relationship we saw previously   If you take a look at the diagram above  you can see that there are two ways to manage this relationship  In the Post entity  you have the comments collection   OneToMany      mappedBy    quot post quot       cascade   CascadeType ALL      orphanRemoval   true   private List lt PostComment gt  comments   new ArrayList lt  gt      And  in the PostComment  the post association is mapped as follows   ManyToOne      fetch   FetchType LAZY    JoinColumn name    quot post id quot   private Post post   Because there are two ways to represent the Foreign Key column  you must define which is the source of truth when it comes to translating the association state change into its equivalent Foreign Key column value modification  MappedBy The mappedBy attribute tells that the  ManyToOne side is in charge of managing the Foreign Key column  and the collection is used only to fetch the child entities and to cascade parent entity state changes to children  e g   removing the parent should also remove the child entities   Synchronize both sides of a bidirectional association Now  even if you defined the mappedBy attribute and the child-side  ManyToOne association manages the Foreign Key column  you still need to synchronize both sides of the bidirectional association  The best way to do that is to add these two utility methods  public void addComment PostComment comment        comments add comment       comment setPost this      public void removeComment PostComment comment        comments remove comment       comment setPost null      The addComment and removeComment methods ensure that both sides are synchronized  So  if we add a child entity  the child entity needs to point to the parent and the parent entity should have the child contained in the child collection

User · Answer

You started with ManyToOne mapping   then you put OneToMany mapping as well for BiDirectional way  Then at OneToMany side  usually your parent table class   you have to mention  mappedBy   mapping is done by and in child table class   so hibernate will not create EXTRA mapping table in DB  like TableName   parent child

User · Answer

By specifying the  JoinColumn on both models you don t have a two way relationship   You have two one way relationships  and a very confusing mapping of it at that   You re telling both models that they  own  the IDAIRLINE column   Really only one of them actually should   The  normal  thing is to take the  JoinColumn off of the  OneToMany side entirely  and instead add mappedBy to the  OneToMany    OneToMany cascade   CascadeType ALL  mappedBy  airline   public Set lt AirlineFlight gt  getAirlineFlights         return airlineFlights      That tells Hibernate  Go look over on the bean property named  airline  on the thing I have a collection of to find the configuration

User · Answer

mappedby  object of entity of same class created in another class     Note -Mapped by can be used only in one class because one table must contain foreign key constraint    if mapped by can be applied on both side then it remove foreign key from both table  and without foreign key there is no relation b w two tables   Note - it can be use for following  annotations - 1  OneTone  2  OneToMany 3  ManyToMany  Note---It cannot be use for following  annotation  - 1  ManyToOne  In one to one  - Perform at any side of mapping but perform at  only one side   It will remove the extra column of foreign key constraint on the table on which class it is applied   For eg   If we apply mapped by in Employee class on employee object then     foreign key from Employee table will be removed

User · Answer

mappedby speaks for itself  it tells hibernate not to map this field  it s already mapped by this field  name  field    field is in the other entity  name of the variable in the class not the table in the database        If you don t do that  hibernate will map this two relation as it s not   the same relation   so we need to tell hibernate to do the mapping in one side only and co-ordinate between them

User · Answer

MappedBy signals hibernate that the key for the relationship is on the other side   This means that although you link 2 tables together  only 1 of those tables has a foreign key constraint to the other one  MappedBy allows you to still link from the table not containing the constraint to the other table

[java] Can someone explain mappedBy in JPA and Hibernate?

Table relationship vs. entity relationship

MappedBy

Synchronize both sides of a bidirectional association

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