I'm developing a REST API (blog) using Spring Boot running with MySQL database.
Simply, I need to sort Article by the number of likes it has. The information about number of likes is stored in article_likes table which acts like association between Article and User (who liked the article).
#Entity
class Article {
#ManyToMany
#JoinTable(
name = "article_likes",
joinColumns = #JoinColumn(name = "article_id", referencedColumnName = "id"),
inverseJoinColumns = #JoinColumn(name = "user_id", referencedColumnName = "id")
)
Set<User> likedBy = new HashSet<>();
#Column(name = "created")
#CreationTimeStamp
LocalDateTime added;
}
I'm having a service which returns a Page of Article for given Pageable (comes from frontend):
Page<Article> getArticles(Pageable pageable);
In frontend, when I'm showing list/page of articles, I'm using following call to get the last (newest) 5 articles.
localhost:8080/api/articles?size=5&sort=added,desc
I want to do the same with but with number of likes, something like this:
localhost:8080/api/articles?size=5&sort=likedBy,desc
I've tried following variations without the success.
sort=likedBy.size
sort=likedBy.count
sort=likedBy.length
with following error:
No property * found for type User! Traversed path: Article.likedBy.",
Is something like this possible? If not, is there a way to store just the read-only count of likes in Article (as attribute) instead? Because in this particular situation its not important who liked it, but only the count.
class Article {
#Transient
#ManyToMany
???
long likes;
}
Related
I have a simple project that has a User model, Sports team model and a Many To Many table where a user can "like" the sports team.
User
#ManyToMany(fetch = FetchType.LAZY)
#JoinTable(
name = "likes",
joinColumns = #JoinColumn(name = "user_id"),
inverseJoinColumns = #JoinColumn(name = "team_id")
)
private List<Team> teamsLiked;
Team
#Id
#GeneratedValue(strategy = GenerationType.IDENTITY)
private Long id;
#NotBlank
#Size(min=2, max=30)
private String teamName;
#NotBlank
private String city;
private String sport;
#ManyToMany(fetch = FetchType.LAZY)
#JoinTable(
name = "likes",
joinColumns = #JoinColumn(name = "team_id"),
inverseJoinColumns = #JoinColumn(name = "user_id")
)
private List<User> likers;
My problem is, when I'm using Spring MVC forms for a user to edit a team, upon submission it completely wipes out existing likes on the Team object under likers. On the edit page, I am using #ModelAttribute and pre populating the existing team object, and have tried to put the likers as a hidden attribute so the data will persist, but that throws an error. I've tried on the #PostMapping backend, to set the origin list of likers before re-saving the DB and that's not working either. Besides using Normal HTML forms to update an object, is there a way I can have the list of users who liked a team persist after updating? Thanks in advance.
What you need here is a DTO and map that onto an existing entity. I think this is a perfect use case for Blaze-Persistence Entity Views.
I created the library to allow easy mapping between JPA models and custom interface or abstract class defined models, something like Spring Data Projections on steroids. The idea is that you define your target structure(domain model) the way you like and map attributes(getters) via JPQL expressions to the entity model.
A DTO model for your use case could look like the following with Blaze-Persistence Entity-Views:
#EntityView(Team.class)
#UpdatableEntityView
public interface TeamDto {
#IdMapping
Long getId();
String getTeamName();
void setTeamName(String teamName);
String getCity();
void setCity(String city);
String getSport();
void setSport(String sport);
}
Querying is a matter of applying the entity view to a query, the simplest being just a query by id.
TeamDto a = entityViewManager.find(entityManager, TeamDto.class, id);
The Spring Data integration allows you to use it almost like Spring Data Projections: https://persistence.blazebit.com/documentation/entity-view/manual/en_US/index.html#spring-data-features
Page<TeamDto> findAll(Pageable pageable);
The best part is, it will only fetch the state that is actually necessary!
And in your case of saving data, you can use the Spring WebMvc integration
that would look something like the following:
#Transactional
#PostMapping("/teams")
void save(#RequestBody TeamDto dto){
repository.save(dto);
}
I have a Parent User Class that has multiple ManyToMany Relationships.
#Table(name = "user")
public class User {
..
#ManyToMany(fetch = FetchType.LAZY, cascade = {CascadeType.MERGE, CascadeType.DETACH})
#JoinTable(
name = "user_address",
joinColumns = { #JoinColumn(name = "user_id")},
inverseJoinColumns = { #JoinColumn(name = "address_id")}
)
#JsonIgnore
private final List<Address> addresses = new ArrayList<Address>();
#ManyToMany(fetch = FetchType.LAZY, cascade = {CascadeType.MERGE, CascadeType.DETACH})
#JoinTable(
name = "reports",
joinColumns = { #JoinColumn(name = "user_id")},
inverseJoinColumns = { #JoinColumn(name = "reports_id")}
)
#JsonIgnore
private final List<Reports> reports = new ArrayList<Reports>();
}
When I access the FIRST ManyToMany property, everything works fine. However, immediately after
accessing the first, when I try to access the SECOND ManyToMany Property I get the "could not initialize proxy - no Session" exception:
#Component
public class Combiner {
public void combineData() {
...
List<Address> addresses = user.getAddress(); // This works
List<Reports> reports = user.getReports(); // Get the error here
..
}
}
The Address and Reports classes have the inverse relationship as many ManyToMany back to the User Entity Above.
public class Address {
#ManyToMany(mappedBy = "addresses", fetch = FetchType.LAZY)
private final List<User> users = new ArrayList<User>();
}
public class Reports {
#ManyToMany(mappedBy = "reports", fetch = FetchType.LAZY)
private final List<User> users = new ArrayList<User>();
}
I tried searching SO for the same error where there are MULTIPLE relationships like mine and the first passes but second fails, but could'nt find a post (or google couldn't understand the search terms, if anyone knows a pre-existing one - please let me know).
Could someone assess what else Im missing?
I've tried these so far to no avail:
Added #Transactional to the parent Service class that calls Combiner above
Made the second failing relationship EAGER. (as i understand it you cant make BOTH EAGER since i get a multiple bags error probably because of Cartesian join)
AM Using SpringBoot (2.2.4) with Hibernate Core {5.4.10.Final}
Approach one:
Make #ManyToMany uni-directional. The exception clearly says it can not initialize the collection of role you have in User class.
As you asked in the comment section Why can't this use case be Bi Directional - You can make this bi-directional as well.
Approach two: make collection of role EAGER or use Hibernate.initialize() to initialize the collection.
Bonus: you can make both collection EAGER by using Set not List.
public class Role {
#ManyToMany
#JoinTable(name = "user_to_role",
joinColumns = #JoinColumn(name = "role_id"),
inverseJoinColumns = #JoinColumn(name = "user_id",referencedColumnName = "id"))
private Set<User> users;
}
public class User {
#ManyToMany
#JoinTable(name = "user_to_role",
joinColumns = #JoinColumn(name = "user_id"),
inverseJoinColumns = #JoinColumn(name = "role_id"))
private Set<Role> roles;
}
I have a many to many relationship between the two classes. When calling role.getUsers(), I want to get only the user ids, the rest of the fields should be ignored, since there will be a lot of data and I don't want to load everything, How can I achieve this?
A straightforward way to do it would be to use a Criteria query, but to use it inside an Entity, you'd have to inject an EntityManager there, which is considered a bad practice. A better solution would be to create this query in a Service.
But if you still want to do it, then your getUsers method would look something like this:
public List<User> getUsers() {
Criteria cr = entityManager.createCriteria(User.class)
.setProjection(Projections.projectionList()
.add(Projections.property("id"), "id")
.setResultTransformer(Transformers.aliasToBean(User.class));
List<User> list = cr.list();
return list;
}
If you want to restrict your list, just use a Restrictions, like so: criteria.add(Restrictions.eq("id", yourRestrictedId))
Since you have mapped the entities User and Role using #ManyToMany relationship, you need to create a DAO/Service class to implement the business logic to filter only userIds and return the same.
This cannot be handled in your Model\Entity classes as it will defy the whole concept of Object-Relational mapping.
I can create the business logic using DAO for your example if you want but you will get 10's of blogs achieving the same.
For your reference,you can check my sample project here.
I need for a project to join 2 SQL tables implemented like this :
I know that I'm not supposed to implement the table IngredientList as an object cause it's only here for SQL structure.
My code goes like this :
#Entity
#Table(name="recipe")
public class Recipe {
#Id
#GeneratedValue(strategy=GenerationType.AUTO)
#Column(name="id_recipe")
private Long id;
#OneToMany
#JoinTable(name="liste_ingredients", joinColumns = #JoinColumn(name = "id_recette",
referencedColumnName = "id_recette"),
inverseJoinColumns = #JoinColumn(name = "id_ingredient",
referencedColumnName = "id_ingredient"))
List<Ingredient> ingredients;
/* Getter/Setter/Constructor */
}
Which is the classic way but with that I lose the Quantity attribute that I want to associate with ingredient. And I don't get how I can work around this without creating an object IngredientList.
Thanks in advance.
Nevermind that I got my answer gonna edit it soon with code, for anyone with the same question.
I am using spring with JPA and I have 2 model classes
User
Resources
I have defined many to many relation using -
#ManyToMany(cascade = CascadeType.ALL, fetch = FetchType.LAZY)
#JoinTable(name = "users_resources", joinColumns = #JoinColumn(name = "user_id", referencedColumnName = "id"), inverseJoinColumns = #JoinColumn(name = "resource_id", referencedColumnName = "id"))
This definition has created another table users_resources.
To insert data in users_resources table, the REST api is sending just the User id [primary key] and Resource Id [primary key].
Is there anyway I can insert data into users_resources without fetching full user object and resource object? I wanted to use native query but it seems PersistenceJPAConfig has noway to run native INSERT query.
If you know Id of the object, and you need object reference (without object details) you could use EntityManager.getReference()
User userReference=em.getReference(User.class,userId);
Resource resourceReference=em.getReference(Resource.class,resourceId);
Using references will not exectue any SELECTs from the database.