JPA Criteria API - How to add JOIN clause as general sentence as possible

Question

I am trying to construct queries dynamically  and my next target is add JOIN clauses  I don t know how can I use the API    By now  for example  this code work for me        Class baseClass         CriteriaBuilder cb   JpaHandle get   getCriteriaBuilder    CriteriaQuery cq   cb createQuery this baseClass   Root entity    cq from this baseClass    Predicate restrictions   null      restrictions   cb conjunction    restrictions   cb and restrictions  entity  get  id   in this listId        cq where restrictions       Query qry   JpaHandle get   createQuery cq      Note   JpaHandle is from wicket-JPA implementation   My desire is add JOIN clause  as generical as possible     I have the particular annotations in the classes  this baseClass    For example      ManyToOne fetch   FetchType LAZY   JoinColumn name    assay id   nullable   false    So Is there a way to something like this in standard JPA    Note   this don t compile   Here a practical fail aproaches         Join lt Experiment Assay gt  experimentAssays   entity  join  entity  get  assay id        Or like that         CriteriaQuery lt Customer gt  q   cb createQuery Customer class   Root lt Customer gt  c   q from Customer class   SetJoin lt Customer  PurchaseOrder gt  o   c join Customer  orders     For me  if it could be more generical as possible it will be great            Join joinClause   entity join entity  get  assay id    entity2  get  id       Of course  I have the particular annotations in the classes  this baseClass    Thank you for your time  I ll appreciate all kind of comments

User · Answer

You don t need to learn JPA  You can use my easy-criteria for JPA2  https   sourceforge net projects easy-criteria files    Here is the example  CriteriaComposer lt Pet gt  petCriteria CriteriaComposer from Pet class   where Pet  type  EQUAL   Cat   join Pet  owner  where Ower  name EQUAL   foo     List lt Pet gt  result   CriteriaProcessor findAllEntiry petCriteria     OR  List lt Tuple gt  result    CriteriaProcessor findAllTuple petCriteria

User · Answer

Actually you don t have to deal with the static metamodel if you had your annotations right   With the following entities      Entity public class Pet      Id   protected Long id    protected String name    protected String color     ManyToOne   protected Set lt Owner gt  owners      Entity public class Owner      Id   protected Long id    protected String name      You can use this    CriteriaQuery lt Pet gt  cq   cb createQuery Pet class   Metamodel m   em getMetamodel    EntityType lt Pet gt  petMetaModel   m entity Pet class    Root lt Pet gt  pet   cq from Pet class   Join lt Pet  Owner gt  owner   pet join petMetaModel getSet  owners   Owner class

User · Answer

Maybe the following extract from the Chapter 23 - Using the Criteria API to Create Queries  of the Java EE 6 tutorial will throw some light  actually  I suggest reading the whole Chapter 23       Querying Relationships Using Joins      For queries that navigate to related   entity classes  the query must define   a join to the related entity by   calling one of the From join methods   on the query root object  or another   join object  The join methods are   similar to the JOIN keyword in JPQL       The target of the join uses the   Metamodel class of type   EntityType lt T gt  to specify the   persistent field or property of the   joined entity       The join methods return an object of   type Join lt X  Y gt   where X is the   source entity and Y is the target of   the join       Example 23-10 Joining a Query  CriteriaQuery lt Pet gt  cq   cb createQuery Pet class   Metamodel m   em getMetamodel    EntityType lt Pet gt  Pet    m entity Pet class    Root lt Pet gt  pet   cq from Pet class   Join lt Pet  Owner gt  owner   pet join Pet  owners         Joins can be chained together to   navigate to related entities of the   target entity without having to create   a Join lt X  Y gt  instance for each join       Example 23-11 Chaining Joins Together   in a Query  CriteriaQuery lt Pet gt  cq   cb createQuery Pet class   Metamodel m   em getMetamodel    EntityType lt Pet gt  Pet    m entity Pet class   EntityType lt Owner gt  Owner    m entity Owner class    Root lt Pet gt  pet   cq from Pet class   Join lt Owner  Address gt  address   cq join Pet  owners  join Owner  addresses      That being said  I have some additional remarks   First  the following line in your code   Root entity    cq from this baseClass     Makes me think that you somehow missed the Static Metamodel Classes part  Metamodel classes such as Pet  in the quoted example are used to describe the meta information of a persistent class  They are typically generated using an annotation processor  canonical metamodel classes  or can be written by the developer  non-canonical metamodel   But your syntax looks weird  I think you are trying to mimic something that you missed   Second  I really think you should forget this assay id foreign key  you re on the wrong path here  You really need to start to think object and association  not tables and columns   Third  I m not really sure to understand what you mean exactly by adding a JOIN clause as generical as possible and what your object model looks like  since you didn t provide it  see previous point   It s thus just impossible to answer your question more precisely   To sum up  I think you need to read a bit more about JPA 2 0 Criteria and Metamodel API and I warmly recommend the resources below as a starting point   See also   the section 6 2 1 Static Metamodel Classes in the JPA 2 0 specification Dynamic  typesafe queries in JPA 2 0 Using the Criteria API and Metamodel API to Create Basic Type-Safe Queries    Related question   How to generate JPA 2 0 metamodel

User · Answer

Warning   There s a numbers of errors on the Sun JPA 2 example and the resulting pasted content in Pascal s answer  Please consult this post   This post and the Sun Java EE 6 JPA 2 example really held back my comprehension of JPA 2   After plowing through the Hibernate and OpenJPA manuals and thinking that I had a good understanding of JPA 2  I still got confused afterwards when returning to this post

[java] JPA Criteria API - How to add JOIN clause (as general sentence as possible)

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