How to map a composite key with JPA and Hibernate

Question

In this code  how to generate a Java class for the composite key  how to composite key in hibernate    create table Time        levelStation int 15  not null       src varchar 100  not null       dst varchar 100  not null       distance int 15  not null       price int 15  not null       confPathID int 15  not null       constraint ConfPath fk foreign key confPathID  references ConfPath confPathID        primary key  levelStation  confPathID    ENGINE InnoDB DEFAULT CHARSET utf8

User · Answer

Another option is to map is as a Map of composite elements in the ConfPath table.

This mapping would benefit from an index on (ConfPathID,levelStation) though.

public class ConfPath {
    private Map<Long,Time> timeForLevelStation = new HashMap<Long,Time>();

    public Time getTime(long levelStation) {
        return timeForLevelStation.get(levelStation);
    }

    public void putTime(long levelStation, Time newValue) {
        timeForLevelStation.put(levelStation, newValue);
    }
}

public class Time {
    String src;
    String dst;
    long distance;
    long price;

    public long getDistance() {
        return distance;
    }

    public void setDistance(long distance) {
        this.distance = distance;
    }

    public String getDst() {
        return dst;
    }

    public void setDst(String dst) {
        this.dst = dst;
    }

    public long getPrice() {
        return price;
    }

    public void setPrice(long price) {
        this.price = price;
    }

    public String getSrc() {
        return src;
    }

    public void setSrc(String src) {
        this.src = src;
    }
}

Mapping:

<class name="ConfPath" table="ConfPath">
    <id column="ID" name="id">
        <generator class="native"/>
    </id>
    <map cascade="all-delete-orphan" name="values" table="example"
            lazy="extra">
        <key column="ConfPathID"/>
        <map-key type="long" column="levelStation"/>
        <composite-element class="Time">
            <property name="src" column="src" type="string" length="100"/>
            <property name="dst" column="dst" type="string" length="100"/>
            <property name="distance" column="distance"/>
            <property name="price" column="price"/>
        </composite-element>
    </map>
</class>

User · Answer

Let s take a simple example  Let s say two tables named test and customer are there described as   create table test    test id int 11  not null auto increment    primary key test id     create table customer    customer id int 11  not null auto increment    name varchar 50  not null    primary key customer id      One more table is there which keeps the track of tests and customer   create table tests purchased    customer id int 11  not null    test id int 11  not null    created date datetime not null    primary key customer id  test id      We can see that in the table tests purchased the primary key is a composite key  so we will use the  lt composite-id     gt     lt  composite-id gt  tag in the hbm xml mapping file  So the PurchasedTest hbm xml will look like    lt  xml version  1 0   gt   lt  DOCTYPE hibernate-mapping PUBLIC    -  Hibernate Hibernate Mapping DTD 3 0  EN     http   hibernate sourceforge net hibernate-mapping-3 0 dtd  gt    lt hibernate-mapping gt     lt class name  entities PurchasedTest  table  tests purchased  gt        lt composite-id name  purchasedTestId  gt         lt key-property name  testId  column  TEST ID    gt         lt key-property name  customerId  column  CUSTOMER ID    gt       lt  composite-id gt        lt property name  purchaseDate  type  timestamp  gt         lt column name  created date    gt       lt  property gt      lt  class gt   lt  hibernate-mapping gt    But it doesn t end here  In Hibernate we use session load  entityClass  id type object  to find and load the entity using primary key  In case of composite keys  the ID object should be a separate ID class  in above case a PurchasedTestId class  which just declares the primary key attributes like below   import java io Serializable   public class PurchasedTestId implements Serializable     private Long testId    private Long customerId        an easy initializing constructor   public PurchasedTestId Long testId  Long customerId        this testId   testId      this customerId   customerId         public Long getTestId         return testId         public void setTestId Long testId        this testId   testId         public Long getCustomerId         return customerId         public void setCustomerId Long customerId        this customerId   customerId          Override   public boolean equals Object arg0        if arg0    null  return false      if   arg0 instanceof PurchasedTestId   return false      PurchasedTestId arg1    PurchasedTestId  arg0      return  this testId longValue      arg1 getTestId   longValue     amp  amp              this customerId longValue      arg1 getCustomerId   longValue             Override   public int hashCode         int hsCode      hsCode   testId hashCode        hsCode   19   hsCode  customerId hashCode        return hsCode          Important point is that we also implement the two functions hashCode   and equals   as Hibernate relies on them

User · Answer

You need to use  EmbeddedId    Entity class Time        EmbeddedId     TimeId id       String src      String dst      Integer distance      Integer price      Embeddable class TimeId implements Serializable       Integer levelStation      Integer confPathID

User · Answer

Using hbm xml        lt composite-id gt            lt  -- lt key-many-to-one name  productId  class  databaselayer users UserDB  column  user name   gt -- gt           lt key-property name  productId  column  PRODUCT Product ID  type  int   gt           lt key-property name  categoryId  column  categories id  type  int    gt       lt  composite-id gt         Using Annotation   Composite Key Class  public  class PK implements Serializable      private int PRODUCT Product ID           private int categories id        public PK int productId  int categoryId            this PRODUCT Product ID   productId          this categories id   categoryId             public int getPRODUCT Product ID             return PRODUCT Product ID             public void setPRODUCT Product ID int PRODUCT Product ID            this PRODUCT Product ID   PRODUCT Product ID             public int getCategories id             return categories id             public void setCategories id int categories id            this categories id   categories id             private PK             Override     public boolean equals Object o            if   this    o                 return true                     if   o    null    getClass      o getClass                   return false                     PK pk    PK  o          return Objects equals PRODUCT Product ID  pk PRODUCT Product ID    amp  amp                  Objects equals categories id  pk categories id                Override     public int hashCode             return Objects hash PRODUCT Product ID  categories id              Entity Class   Entity name    product category    IdClass  PK class   public  class ProductCategory implements Serializable        Id         private int PRODUCT Product ID            Id      private int categories id        public ProductCategory int productId  int categoryId            this PRODUCT Product ID   productId           this categories id   categoryId             public ProductCategory            public int getPRODUCT Product ID             return PRODUCT Product ID             public void setPRODUCT Product ID int PRODUCT Product ID            this PRODUCT Product ID   PRODUCT Product ID             public int getCategories id             return categories id             public void setCategories id int categories id            this categories id   categories id             public void setId PK id            this PRODUCT Product ID   id getPRODUCT Product ID            this categories id   id getCategories id               public PK getId             return new PK              PRODUCT Product ID              categories id

User · Answer

Looks like you are doing this from scratch  Try using available reverse engineering tools like Netbeans Entities from Database to at least get the basics automated  like embedded ids   This can become a huge headache if you have many tables  I suggest avoid reinventing the wheel and use as many tools available as possible to reduce coding to the minimum and most important part  what you intent to do

User · Answer

Assuming you have the following database tables   First  you need to create the  Embeddable holding the composite identifier   Embeddable public class EmployeeId implements Serializable          Column name    quot company id quot       private Long companyId         Column name    quot employee number quot       private Long employeeNumber        public EmployeeId                 public EmployeeId Long companyId  Long employeeId            this companyId   companyId          this employeeNumber   employeeId              public Long getCompanyId             return companyId              public Long getEmployeeNumber             return employeeNumber               Override     public boolean equals Object o            if  this    o  return true          if    o instanceof EmployeeId   return false          EmployeeId that    EmployeeId  o          return Objects equals getCompanyId    that getCompanyId     amp  amp                  Objects equals getEmployeeNumber    that getEmployeeNumber                  Override     public int hashCode             return Objects hash getCompanyId    getEmployeeNumber              With this in place  we can map the Employee entity which uses the composite identifier by annotating it with  EmbeddedId   Entity name    quot Employee quot    Table name    quot employee quot   public class Employee          EmbeddedId     private EmployeeId id        private String name        public EmployeeId getId             return id              public void setId EmployeeId id            this id   id              public String getName             return name              public void setName String name            this name   name           The Phone entity which has a  ManyToOne association to Employee  needs to reference the composite identifier from the parent class via two  JoinColumnmappings   Entity name    quot Phone quot    Table name    quot phone quot   public class Phone          Id      Column name    quot  number  quot       private String number         ManyToOne      JoinColumns            JoinColumn              name    quot company id quot               referencedColumnName    quot company id quot             JoinColumn              name    quot employee number quot               referencedColumnName    quot employee number quot              private Employee employee        public Employee getEmployee             return employee              public void setEmployee Employee employee            this employee   employee              public String getNumber             return number              public void setNumber String number            this number   number

User · Answer

To map a composite key  you can use the EmbeddedId or the IdClass annotations  I know this question is not strictly about JPA but the rules defined by the specification also applies  So here they are      2 1 4 Primary Keys and Entity Identity               A composite primary key must   correspond to either a single   persistent field or property or to a   set of such fields or properties as   described below  A primary key class   must be defined to represent a   composite primary key  Composite   primary keys typically arise when   mapping from legacy databases when the   database key is comprised of several   columns  The EmbeddedId and   IdClass annotations are used to   denote composite primary keys  See   sections 9 1 14 and 9 1 15                The following rules apply for   composite primary keys          The primary key class must be public and must have a public no-arg   constructor    If property-based access is used  the properties of the primary key   class must be public or protected    The primary key class must be serializable     The primary key class   must define equals and hashCode   methods  The semantics of value   equality for these methods must be   consistent with the database equality   for the database types to which the   key is mapped    A composite primary key must either be represented and mapped as an   embeddable class  see Section 9 1 14       EmbeddedId Annotation     or must be   represented and mapped to multiple   fields or properties of the entity   class  see Section 9 1 15     IdClass   Annotation        If the composite primary key class is mapped to multiple fields or   properties of the entity class  the   names of primary key fields or   properties in the primary key class   and those of the entity class must   correspond and their types must be the   same       With an IdClass  The class for the composite primary key could look like  could be a static inner class    public class TimePK implements Serializable       protected Integer levelStation      protected Integer confPathID       public TimePK           public TimePK Integer levelStation  Integer confPathID            this levelStation   levelStation          this confPathID   confPathID               equals  hashCode     And the entity    Entity  IdClass TimePK class  class Time implements Serializable        Id     private Integer levelStation       Id     private Integer confPathID       private String src      private String dst      private Integer distance      private Integer price          getters  setters     The IdClass annotation maps multiple fields to the table PK    With EmbeddedId  The class for the composite primary key could look like  could be a static inner class     Embeddable public class TimePK implements Serializable       protected Integer levelStation      protected Integer confPathID       public TimePK           public TimePK Integer levelStation  Integer confPathID            this levelStation   levelStation          this confPathID   confPathID               equals  hashCode     And the entity    Entity class Time implements Serializable        EmbeddedId     private TimePK timePK       private String src      private String dst      private Integer distance      private Integer price                 The  EmbeddedId annotation maps a PK class to table PK   Differences    From the physical model point of view  there are no differences  EmbeddedId somehow communicates more clearly that the key is a composite key and IMO makes sense when the combined pk is either a meaningful entity itself or it reused in your code   IdClass is useful to specify that some combination of fields is unique but these do not have a special meaning    They also affect the way you write queries  making them more or less verbose     with IdClass  select t levelStation from Time t  with EmbeddedId  select t timePK levelStation from Time t    References   JPA 1 0 specification   Section 2 1 4  Primary Keys and Entity Identity  Section 9 1 14  EmbeddedId Annotation  Section 9 1 15  IdClass Annotation

User · Answer

The primary key class must define equals and hashCode methods     When implementing equals you should use instanceof to allow comparing with subclasses  If Hibernate lazy loads a one to one or many to one relation  you will have a proxy for the class instead of the plain class  A proxy is a subclass  Comparing the class names would fail  More technically  You should follow the Liskows Substitution Principle and ignore symmetricity    The next pitfall is using something like name equals that name  instead of name equals that getName     The first will fail  if that is a proxy      http   www laliluna de jpa-hibernate-guide ch06s06 html

[java] How to map a composite key with JPA and Hibernate?

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