JPA How to get entity based on field value other than ID

Question

In JPA  Hibernate   when we automatically generate the ID field  it is assumed that the user has no knowledge about this key  So  when obtaining the entity  user would query based on some field other than ID  How do we obtain the entity in that case  since em find   cannot be used     I understand we can use a query and filter the results later  But  is there a more direct way  because this is a very common problem as I understand

User · Answer

This solution is from Beginning Hibernate book         Query lt User gt  query   session createQuery  from User u where u scn  scn   User class        query setParameter  scn   scn        User user   query uniqueResult

User · Answer

Basically  you should add a specific unique field  I usually use xxxUri fields   class User         Id        automatically generated     private Long id          globally unique id      Column name    SCN   nullable   false  unique   true      private String scn      And you business method will do like this   public User findUserByScn  NotNull final String scn        CriteriaBuilder builder   manager getCriteriaBuilder        CriteriaQuery lt User gt  criteria   builder createQuery User class       Root lt User gt  from   criteria from User class       criteria select from       criteria where builder equal from get User  scn   scn        TypedQuery lt User gt  typed   manager createQuery criteria       try           return typed getSingleResult          catch  final NoResultException nre            return null

User · Answer

It is not a  problem  as you stated it   Hibernate has the built-in find    but you have to build your own query in order to get a particular object  I recommend using Hibernate s Criteria    Criteria criteria   session createCriteria YourClass class   YourObject yourObject   criteria add Restrictions eq  yourField   yourFieldValue                                 uniqueResult      This will create a criteria on your current class  adding the restriction that the column  yourField  is equal to the value yourFieldValue  uniqueResult   tells it to bring a unique result  If more objects match  you should retrive a list   List lt YourObject gt  list   criteria add Restrictions eq  yourField   yourFieldValue   list      If you have any further questions  please feel free to ask  Hope this helps

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Best practice is using  NaturalId annotation  It can be used as the business key for some cases it is too complicated  so some fields are using as the identifier in the real world   For example  I have user class with user id as primary key  and email is also unique field  So we can use email as our natural id     Entity  Table name  user   public class User      Id    Column name  id     private int id      NaturalId    Column name  email     private String email      Column name  name     private String name      To get our record  just simply use  session byNaturalId     Session session   sessionFactory getCurrentSession    User user   session byNaturalId User class                      using  email   huchenhai qq com                       load

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Refer - Spring docs for query methods  We can add methods in Spring Jpa by passing diff params in methods like  List lt Person gt  findByEmailAddressAndLastname EmailAddress emailAddress  String lastname         Enabling static ORDER BY for a query   List lt Person gt  findByLastnameOrderByFirstnameAsc String lastname

User · Answer

Have a look at    JPA query language  The Java Persistence Query Language   JPA Criteria API   Using the Criteria API to Create Queries

User · Answer

I ve written a library that helps do precisely this  It allows search by object simply by initializing only the fields you want to filter by  https   github com kg6zvp GenericEntityEJB

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Edit  Just realized that  Chinmoy was getting at basically the same thing  but I think I may have done a better job ELI5     If you re using a flavor of Spring Data to help persist   fetch things from whatever kind of Repository you ve defined  you can probably have your JPA provider do this for you via some clever tricks with method names in your Repository interface class   Allow me to explain    As a disclaimer  I just a few moments ago did still am figuring this out for myself     For example  if I am storing Tokens in my database  I might have an entity class that looks like this    Data     lt  lt  Project Lombok convenience annotation  Entity public class Token        Id      Column name    TOKEN ID       private String tokenId        Column name    TOKEN       private String token        Column name    EXPIRATION       private String expiration        Column name    SCOPE       private String scope      And I probably have a CrudRepository lt K V gt  interface defined like this  to give me simple CRUD operations on that Repository for free    Repository    CrudRepository lt  Entity Type    Entity Primary Key Type  gt  public interface TokenRepository extends CrudRepository lt Token  String gt        And when I m looking up one of these tokens  my purpose might be checking the expiration or scope  for example   In either of those cases  I probably don t have the tokenId handy  but rather just the value of a token field itself that I want to look up     To do that  you can add an additional method to your TokenRepository interface in a clever way to tell your JPA provider that the value you re passing in to the method is not the tokenId  but the value of another field within the Entity class  and it should take that into account when it is generating the actual SQL that it will run against your database     Repository    CrudRepository lt  Entity Type    Entity Primary Key Type  gt  public interface TokenRepository extends CrudRepository lt Token  String gt         List lt Token gt  findByToken String token       I read about this on the Spring Data R2DBC docs page  and it seems to be working so far within a SpringBoot 2 x app storing in an embedded H2 database

User · Answer

Write a custom method like this        public Object findByYourField Class entityClass  String yourFieldValue        CriteriaBuilder criteriaBuilder   entityManager getCriteriaBuilder        CriteriaQuery lt Object gt  criteriaQuery   criteriaBuilder createQuery entityClass       Root lt Object gt  root   criteriaQuery from entityClass       criteriaQuery select root        ParameterExpression lt String gt  params   criteriaBuilder parameter String class       criteriaQuery where criteriaBuilder equal root get  yourField    params         TypedQuery lt Object gt  query   entityManager createQuery criteriaQuery       query setParameter params  yourFieldValue        List lt Object gt  queryResult   query getResultList         Object returnObject   null       if  CollectionUtils isNotEmpty queryResult             returnObject   queryResult get 0              return returnObject

User · Answer

No  you don t need to make criteria query it would be boilerplate code you just do simple thing if you working in Spring-boot  in your repo declare a method name with findBy exact field name   Example-  if your model or document consist a string field myField  and you want to find by it then your method name will be   findBymyField String myField

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In my Spring Boot app I resolved a similar type of issue like this          Autowired private EntityManager entityManager   public User findByEmail String email        User user   null      Query query   entityManager createQuery  SELECT u FROM User u WHERE u email  email        query setParameter  email   email       try           user    User  query getSingleResult          catch  Exception e               Handle exception           return user

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if you have repository for entity Foo and need to select all entries with exact string value boo  also works for other primitive types or entity types   Put this into your repository interface   List lt Foo gt  findByBoo String boo     if you need to order results   List lt Foo gt  findByBooOrderById String boo     See more at reference

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Using CrudRepository and JPA query works for me    import org springframework data jpa repository Query  import org springframework data repository CrudRepository  import org springframework data repository query Param   public interface TokenCrudRepository extends CrudRepository lt Token  Integer gt             Finds a token by using the user as a search criteria      param user     return  A token element matching with the given user           Query  SELECT t FROM Token t WHERE LOWER t user    LOWER  user        public Token find  Param  user   String user        and you invoke the find custom method like this   public void destroyCurrentToken String user       AbstractApplicationContext context   getContext         repository   context getBean TokenCrudRepository class        Token token     TokenCrudRepository  repository  find user        int idToken   token getId         repository delete idToken        context close

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All the answers require you to write some sort of SQL HQL whatever  Why  You don t have to - just use CriteriaBuilder   Person java    Entity class Person       Id  GeneratedValue   private int id      Column name    name     private String name     Column name    age     private int age            Dao java   public class Dao      public static Person getPersonByName String name             SessionFactory sessionFactory   new Configuration   configure   buildSessionFactory            Session session   sessionFactory openSession            session beginTransaction             CriteriaBuilder cb   session getCriteriaBuilder             CriteriaQuery lt Person gt  cr   cb createQuery Person class           Root lt Person gt  root   cr from Person class           cr select root  where cb equal root get  name    name       here you pass a class field  not a table column  in this example they are called the same           Query lt Person gt  query   session createQuery cr           query setMaxResults 1           List lt Person gt  result   query getResultList            session close             return result get 0           example of use   public static void main String   args       Person person   Dao getPersonByName  John      System out println person getAge        John s age

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I solved a similar problem  where I wanted to find a book by its isbnCode not by your id primary key    Entity public class Book implements Serializable       private static final long serialVersionUID   1L        Id      GeneratedValue strategy GenerationType IDENTITY      private Integer id      private String isbnCode        In the repository the method was created like  kamalveer singh mentioned  Note that the method name is findBy fieldName  in my case  findByisbnCode    Repository public interface BookRepository extends JpaRepository lt Book  Integer gt         Book findByisbnCode String isbnCode      Then  implemented the method in the service   Service public class BookService         Autowired     private BookRepository repo                public Book findByIsbnCode String isbnCode            Book obj   repo findByisbnCode isbnCode           return obj

[java] JPA: How to get entity based on field value other than ID?

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