JPA entity without id

Question

I have a database with the following structure   CREATE TABLE entity       id SERIAL      name VARCHAR 255       PRIMARY KEY  id      CREATE TABLE entity property       entity id SERIAL      name VARCHAR 255       value TEXT      When I try to create an EntityProperty class   Entity  Table name  entity property   public class EntityProperty        private String name      private String value        Column name  name       public String getName             return name            public void setName String name            this name   name              Column name  value   nullable true  length 255      public String getValue             return value            public void setValue String value            this value   value            I get the following exception   org hibernate AnnotationException  No identifier specified for entity  package EntityProperty   I know that JPA entities must have a primary key but I can t change the database schema due to reasons that are beyond my control  Is it possible to create JPA  Hibernate  entities that will work with database schema like this

User · Answer

I guess you can use  CollectionOfElements  for hibernate jpa 1     ElementCollection  jpa 2  to map a collection of  entity properties  to a List in entity   You can create the EntityProperty type and annotate it with  Embeddable

User · Answer

I guess your entity property has a composite key  entity id  name  where entity id is a foreign key to entity  If so  you can map it as follows    Embeddable public class EntityPropertyPK        Column name    name       private String name        ManyToOne      JoinColumn name    entity id       private Entity entity               Entity   Table name  entity property    public class EntityProperty         EmbeddedId     private EntityPropertyPK id        Column name    value       private String value

User · Answer

See the Java Persistence book  Identity and Sequencing  The relevant part for your question is the No Primary Key section      Sometimes your object or table has no primary key   The best solution   in this case is normally to add a generated id to the object and   table   If you do not have this option  sometimes there is a column or   set of columns in the table that make up a unique value   You can use   this unique set of columns as your id in JPA   The JPA Id   does not always have to match the database table primary key   constraint  nor is a primary key or a unique constraint required       If your table truly has no unique columns  then use all of the columns   as the id   Typically when this occurs the data is read-only  so even   if the table allows duplicate rows with the same values  the objects   will be the same anyway  so it does not matter that JPA thinks they   are the same object   The issue with allowing updates and deletes is   that there is no way to uniquely identify the object s row  so all of   the matching rows will be updated or deleted       If your object does not have an id  but its  table does  this is fine    Make the object an Embeddable object  embeddable objects   do not have ids   You will need a Entity that contains   this Embeddable to persist and query it

User · Answer

If there is a one to one mapping between entity and entity property you can use entity id as the identifier

User · Answer

I know that JPA entities must have primary key but I can t change database structure due to reasons beyond my control    More precisely  a JPA entity must have some Id defined  But a JPA Id does not necessarily have to be mapped on the table primary key  and JPA can somehow deal with a table without a primary key or unique constraint        Is it possible to create JPA  Hibernate  entities that will be work with database structure like this     If you have a column or a set of columns in the table that makes a unique value  you can use this unique set of columns as your Id in JPA    If your table has no unique columns at all  you can use all of the columns as the Id   And if your table has some id but your entity doesn t  make it an Embeddable

[java] JPA entity without id

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