I'm trying to use stream in java, i had a student class:
#Entity
#Data #AllArgsConstructor #NoArgsConstructor
public class Student {
#Id #GeneratedValue(strategy = GenerationType.IDENTITY)
private int id;
private String name;
I added some students:
Stream.of("John","Sophie","emilia").forEach(s->{
studentRepository.save(new Student(s));
});
The probleme is in the below code:
int[] empIds = { 1, 2, 3 };
List<Student> students= Stream.of(empIds)
.map(studentRepository::findById).collect(Collectors.toList());
i got this error: bad return type in method reference:cannot convert java.util.Optional to R. My IDE underline studentRepository::findById.
Many thanks.
The First problem is Stream.of will create an stream of int arrays instead of stream of Integer
For Example
Stream.of(empIds).forEach(System.out::println); //[I#7c3e4b1a
IntStream.of(empIds).forEach(System.out::println); //1 2 3
So use IntStream.of or Arrays.stream()
If findById() is returning Optional<Student> then use isPresent to process only the Optional objects that contain Student
Arrays.stream
List<Student> students= Arrays.stream(empIds)
.mapToObj(studentRepository::findById)
.filter(Optional::isPresent)
.map(Optional::get)
.collect(Collectors.toList());
IntStream.of
List<Student> students= IntStream.of(empIds)
.mapToObj(studentRepository::findById)
.filter(Optional::isPresent)
.map(Optional::get)
.collect(Collectors.toList());
In current approach your are returning List<Optional<Student>>
List<Optional<Student>> students= IntStream.of(empIds)
.map(studentRepository::findById).collect(Collectors.toList());
To be able to use it in map() function, StudentRepository.findById() needs to return Optional<Student> instead of just Student.
Thank you all, your answers helped me to derive a solution:
List<Student> students= Arrays.stream(empIds)
.mapToObj(id->
studentRepository.findById(id).get())
.collect(Collectors.toList());
And the response of deadpool is great also, we have to add .map(Optional::get) to get the stream of student because studentRepository::findById return stream of optioanl that's why the error.
List<Student> students= Arrays.stream(empIds)
.mapToObj(studentRepository::findById)
// .filter(Optional::isPresent)
.map(Optional::get)
.collect(Collectors.toList());
Related
Trying to wrap my head around the reactor model and pipeline, I want to insert to mongo a couple of Users, then for each user I would like to insert several (10) Offers
My current implementation include inserting 3 users to the database, block and insert the offers (only for 1 user) in a somewhat backward way, like so
Flux.just(u1, u2, u3).flatMap(u -> reactiveMongoTemplate.insert(u)).blockLast();
Arrays.asList(u1, u2, u3).forEach(user -> {
IntStream.range(0,10).forEach(i -> reactiveMongoTemplate.insert(new Offer(user)).subscribe());
});
The first line run fine, but I get the following exception
java.lang.IllegalStateException: state should be: open
Of course I can bypass this by inserting for each user separately, I don't know why this exception was raised and appreciate an answer about this issue as well
My main question is how to write it in the most reactive way, should I need to block in order to populate the entity Id after insert or there is a better way?
The exact implementation of User and Offer doesn't really matter, it can be a any simple records, but here they are
#Data
#AllArgsConstructor
#NoArgsConstructor
#Document(collection = "users")
public class User extends BaseEntity {
private String name;
}
...
#Data
#Document(collection = "offers")
public class Offer extends BaseEntity {
private String title;
#JsonSerialize(using = ToStringSerializer.class)
private ObjectId user;
public Offer(){
this.title = "some title " + new Random().nextInt(10);
}
public Offer(User user){
this();
this.user = new ObjectId(user.getId());
}
public void setUser(String userId) {
this.user = new ObjectId(userId);
}
}
reactiveMongoTemplate is from spring-boot-starter-data-mongodb-reactive #EnableReactiveMongoRepositories
Thx
Turn out I was pretty close to the correct solution
Flux.just(u1, u2, u3).flatMap(u -> reactiveMongoTemplate.insert(u)).subscribe(u -> {
Flux.range(0,10).flatMap(i -> reactiveMongoTemplate.insert(new Offer(u))).subscribe();
});
now the code is truly reactive and it can be seen on the database as well (records are inserted with random order)
I saw the Selecting from Multiple Tables in Spring Data already had the solution for multiple tables.
I would like to know if it is possible to write custom query that has tables with pageable and sorting feature at the same time in Spring JPA/DATA.
SELECT s.service_id, s.name, us.rating_id
FROM services s,
ratings r,
user_services us
where
us.service_id = s.service_id and
us.rating_id = r.rating_id and
us.user_id= ?
;
Thanks for you help in advance.
Sorting feature is under question, but pagination is possible to use.
Assume that we have:
#Entity
public class Service {
#Id
private Long id;
private String name;
//...
}
#Entity
public class UserService {
#Id
private Long id;
#ManyToOne
User user;
#ManyToOne
Service service;
#ManyToOne
Rating rating;
//...
}
Then we create a projection:
public interface ServiceRating {
Long getServiceId();
String getServiceName();
Long getRatingId();
}
And then create a query method supported pagination:
public interface UserServiceRepo extends CrudRepository<UserService, Long> {
#Query("select s.id as serviceId, s.name as serviceName, us.rating.id as ratingId from UserService us join us.service s where us.user.id = ?1")
Page<ServiceRating> getServiceRating(Long userId, Pageable pageable);
}
(Since this query does not contain grouping it's not necessary to use an additional countQuery (see the parameter of #Query)).
Test:
Page<ServiceRating> pages = userServiceRepo.getServiceRating(1L, new PageRequest(0, 10));
assertThat(pages.getContent()).hasSize(10));
UPDATE
Sorting also working perfectly.
Just create a Sort object, specify direction and filed name (from the projection):
Sort sort = new Sort(Sort.Direction.ASC, "serviceName");
userServiceRepo.getServiceRating(1L, new PageRequest(0, 10, sort));
is it possible to have a projection with nested collection with Spring JPA?
I have the following 2 simple entity (to explain the problem)
#Entity
#Table(name = "person")
public class Person implements Serializable {
private Integer id;
private String name;
#OneToMany
private List<Address> addressList = new ArrayList<>();
}
#Entity
#Table(name = "address")
public class Address implements Serializable {
private Integer id;
private String city;
private String street;
}
Is it possible to have a projection of Person with following attributes filled in ? {person.name, address.city}
I might be wrong in semantics of word Projection. but the problem is what i need to achieve. Maybe it is not possible with Projection, but is there another way to achieve the end goal? Named Entity graph perhaps ?
P.S. please suggest a solution for Spring JPA not Spring Jpa REST
thanks in advance
You're right, Entity Graphs serve this exact purpose - control field loading.
Create entity graphs dynamically from the code or annotate target entities with Named Entity Graphs and then just use their name.
Here is how to modify your Person class to use Named Entity Graphs:
#Entity
#Table(name = "person")
#NamedEntityGraph(name = "persion.name.with.city",
attributeNodes = #NamedAttributeNode(value = "addressList", subgraph = "addresses.city"),
subgraphs = #NamedSubgraph(name = "addresses.city", attributeNodes = #NamedAttributeNode("city")))
public class Person implements Serializable {
private Integer id;
private String name;
#OneToMany
private List<Address> addressList;
}
And then when loading your person:
EntityGraph graph = em.getEntityGraph("person.name.with.city");
Map hints = new HashMap();
hints.put("javax.persistence.fetchgraph", graph);
return em.find(Person.class, personId, hints);
The same applies for queries, not only em.find method.
Look this tutorial for more details.
I think that that's not usual scenario of Data JPA usage. But you can achieve your goal with pure JPQL:
SELECT a.street, a.person.name FROM Address a WHERE …
This solution has 2 drawbacks:
It forces you to have bidirectional relationship Address ←→ Person
It returns List
Another solution (and that's preferred JPA way) is to create DTO like this:
class MyPersonDTO {
private String personName;
private List<String> cities;
public MyPersonDTO(String personName, List<Address> adresses) {
this.personName = personName;
cities = adresses
.stream()
.map(Address::getCity)
.collect(Collectors.toList());
}
}
And the execute JPQL query like this:
SELECT NEW package.MyPersonDTO(p.name, p.addressList) FROM Person p WHERE …
Return type will be List<MyPersonDTO> in that case.
Of course you can use any of this solutions inside #Query annotation and it should work.
I have two entities Person and Visit
Person has OneToMany relationship with Visit.
I was wondering if I want to save an new entry of Visit, and interm of using RestController. Is my approach correct? Or is there another way which is more efficient?
So I have the following controller which takes a VisitModel from the RequestBody, is it a correct way to call it like so?
VisitModel has the ID of person, and the needed properties for the Visit entity. I use the ID of person to look up in the personRepository for the related Person entry, whereafter I issue it to a new instance of Visit and then use the visitRepository to save it.
#RequestMapping(value="", method=RequestMethod.POST)
public String checkIn(#RequestBody VisitModel visit) {
Person person = personRepository.findById(visit.personId);
Visit newVisit = new Visit(visit.getCheckIn, person);
visitRepository.save(newVisit);
return "success";
}
The Visit entity looks as following
#Entity
public class Visit {
#Id
#GeneratedValue(strategy=GenerationType.AUTO)
private Long id;
#JsonProperty("check_in")
private Date checkIn;
#JsonProperty("check_out")
private Date checkOut;
#ManyToOne
#JoinColumn(name="personId")
private Person person;
public Visit(Date checkIn, Person person) {
this.checkIn = checkIn;
this.person = person;
}
public Date getCheckIn() {
return checkIn;
}
public void setCheckIn(Date checkIn) {
this.checkIn = checkIn;
}
public Date getCheckOut() {
return checkOut;
}
public void setCheckOut(Date checkOut) {
this.checkOut = checkOut;
}
public Person getPerson() {
return person;
}
}
I want to know of the following approach is correct. Or is there another way which is better?
You don't need to get a Person from the database to associate it with a Visit, of course. Because of, you need to have only id of a Person to save it in the foreign key column personId.
If you use JPA EntityManager
Person person = entityManager.getReference(Person.class, visit.personId);
for Hibernate Session
Person person = session.load(Person.class, visit.personId);
This methods just create a proxy and don't do any database requests.
With Hibernate Session I used new Person(personId) as #MadhusudanaReddySunnapu suggested. Everything worked fine.
What is the difference between EntityManager.find() and EntityManger.getReference()?
Hibernate: Difference between session.get and session.load
Yes, that seems to me to be the standard way to map a bidirectional relationship. EDIT: The personId column points to the "id" field of the Person entity.Eg:
#Id
private Long id;
UPDATE: 1: The VisitModel is a 'DTO' or Data Transfer Object. Any separate package is fine. You could consider putting them into a separate jar, so that anyone using your API (with java) can use the jar to create the data before making the call. 2) The way you save it is fine as far as I can see.
i have a problem with getting List from entity Person using Spring data JPA specifications (because of pagination). I need to get all notes by person but dependency between these two entities is on Person side. I don't know how to create my Predicate because Note doesn't contain any attribute related to Person.
I simply can get List with Persons getter but i can't use this way because i need returned data paginated.
#Entity
public class Person implements Serializable {
#Id
private Long personId;
#OneToMany
#JoinColumn(name = "personId")
private List<Note> notes;
}
#Entity
public class Note implements Serializable {
#Id
private Long noteId;
}
Normally, I would write something like this, but i don't have an attribute person in Note and database can't be remapped at this stage.
public static Specification<Note> notesByPerson(final Long personId) {
return new Specification<Note>() {
#Override
public Predicate toPredicate(final Root<Note> root, final CriteriaQuery<?> query,
final CriteriaBuilder builder) {
final Path<Person> per = root.<Person> get("person");
return builder.equal(per.<Long> get("personId"), personId);
}
};
}
Thank you,
Zdend
Solved..
public static Specification<Note> notesByPerson(final Long personId) {
return new Specification<Note>() {
#Override
public Predicate toPredicate(final Root<Note> noteRoot, final CriteriaQuery<?> query,
final CriteriaBuilder cb) {
final Subquery<Long> personQuery = query.subquery(Long.class);
final Root<Person> person = personQuery.from(Person.class);
final Join<Person, Note> notes = person.join("notes");
personQuery.select(notes.<Long> get("noteId"));
personQuery.where(cb.equal(person.<Long> get("personId"), personId));
return cb.in(noteRoot.get("noteId")).value(personQuery);
}
};
}
I am not sure how to do that with Predicates, as I usually dont use them, but in JPQL (or HQL, which is similar), you can do something like this:
SELECT Note n FROM Person.notes WHERE XXXX
It is basically the same thing as doing this in SQL
SELECT n.noteId FROM person as p JOIN persons_notes pn ON pn.person=p.personId JOIN notes as n ON n.noteId=pn.noteId
I would venture a guess that the Predicate method has similar abilities as described above.