I'm creating an application where one large aspect is the ability for users to share content with friends. I'm trying to represent this in the object model and I'm having trouble getting the association to work properly. I'm using a mapping table that records the friender and the friendee, both of which are represented by the primary key (id) of the user. A user can have many friends, and also be referenced by other users. This is what the schema looks like:
Users:
int user_id (PK)
varchar(32) email
varchar(64) password
Users_Map:
int users_map_id (PK)
int friendee_id (FK references users(user_id))
int friender_id (FK references users(user_id))
And this is how I have the User entity set up:
#Data
#Entity
#Table(name = "users")
public class User extends AbstractPersistable<Long> {
#Id
#Column(name = "user_id")
private Long id;
#Column
private String email;
#Column
private String password;
#OneToMany
#JoinTable(name = "users_map",
joinColumns = { #JoinColumn(name = "friender_id") },
inverseJoinColumns = { #JoinColumn(name = "friendee_id") })
private List<User> friends;
}
I run into the following error when deploying the application:
org.hibernate.AnnotationException: A Foreign key refering
com.x.webapp.data.entity.User from
com.x.webapp.data.entity.User has the wrong number of
column. should be 2
I've tried quite a few other configurations, including adding a "referencedColumnName" attribute to each #JoinColumn, but they have also yielded errors. I'm also not entirely sure whether the schema I currently have is the best way to go about mapping users together.
I appreciate any help!
Removing the extension of AbstractPersistable fixed the problem - that contained an #Id reference and clashed with the #Id reference I put inside of User.
Related
I have 3 sql tables:
Account (ID (BIGINT),...)
Card (ID (BIGINT),...)
RelationAccountCard (ACCOUNT (BIGINT), CARD(BIGINT), QUANTITY(int))
One account can have multiple cards, one card can have multiple account
#Data
#Entity
public class Account {
#Id
#GeneratedValue(strategy = GenerationType.AUTO)
private long id;
#ManyToMany
#JoinTable(
name = "RELATION_ACCOUNT_CARD",
joinColumns = #JoinColumn(name = "ACCOUNT"),
inverseJoinColumns = #JoinColumn(name = "CARD"))
private Set<Card> cardsOwned = new HashSet<>();
}
#Data
#Entity
public class Card {
#Id
private long id;
#JsonIgnore
#ManyToMany(mappedBy = "cardsOwned")
private java.util.Set<Account> accounts;
}
This code above is working if the field "QUANTITY" in relationAccountCard didn't exist. I would like to make the code works knowing that I added the "QUANTITY" field. I tried for hours yesterday without success, do you have an idea of how to do this?
Basically I need to replace private Set<Card> cardsOwned; by private Set<RelationAccountCard> relationAccountCards; but I don't know how to
The solution was to delete the composite primary key. To make (card,account) unique, and to add an ID.
Once it was done, I simply followed the answer of this stackoverflow post:
Mapping many-to-many association table with extra column(s)
The following relationship creates a foreign key mapping
#Entity
public class Department {
#Id
private String name;
//some more fields
}
#Entity
public class Employee {
#Id
private long id;
private String name;
private String designation;
#ManyToOne
#JoinColumn(name = "fk_department_id", foreignKey = #ForeignKey(name="fk_department"))
private Department department;
}
generates:
...CONSTRAINT fk_department FOREIGN KEY (fk_department_id) REFERENCES department (name)
Question: how can I trigger this constraint creation in hibernate without having to create the Department entity?
Eg just adding the foreign key #Id field without an explicit entity reference. But still trigger the fk on initial creation. The following is of course invalid:
#ManyToOne
#JoinColumn(name = "fk_department_id", foreignKey = #ForeignKey(name="fk_department"))
private String department;
You get the intention. Is that possible?
(sidenote: I'm not interested in creating that foreign key link by startup ddl/sql statements).
You'll have to drop #ManyToOne at least, since that's for entities.
The following should work by overriding the column definition to include the foreign key while creating it
#Column(name = "department_id", columnDefinition = "VARCHAR(255), foreign key (department_id) REFERENCES department(name)")
private String department;
Now there's only a column and a constraint defined, but no relation (as far as Hibernate knows) defined between entities.
Essentially copied from Hibernate and JPA: how to make a foreign key constraint on a String but that was darn hard to find, so I'm not just going to close this as a duplicate! ;)
Building a Spring Boot REST service backed by MySQL here. I'm adding a super-simple chat feature to an app and this service will handle its backend/enndpoints. I'm new to JPA and have two concerns: (1) that my primordial data model itself may be a little awry; and (2) that I'm not wrapping that model correctly using JPA conventions/best practices.
So first: an overview of the simple problem I'm trying to solve: Users can send Messages to 1+ other Users. This creates a Conversation, which is really just a container of 1+ Messages. If the Conversation is only between 2 Users, it's considered (by the app) to be a Direct Message (DM). Otherwise its considered to be a Group Chat.
My tables (pseudo-schema):
[users]
=======
id PRIMARY KEY AUTO_INC INT NOT NULL,
username VARCHAR(255) NOT NULL
[conversations]
===============
id PRIMARY KEY AUTO_INC INT NOT NULL,
created_on DATETIME NOT NULL
[messages]
==========
id PRIMARY KEY AUTO_INC INT NOT NULL,
conversation_id FOREIGN KEY INT NOT NULL, # on conversations table
sender_id FOREIGN KEY INT NOT NULL, # on users table
text VARCHAR(2000) NOT NULL,
sent_at DATETIME
[users_x_conversations]
=======================
id PRIMARY KEY AUTO_INC INT NOT NULL,
conversation_id FOREIGN KEY INT NOT NULL, # on conversations table
user_id FOREIGN KEY INT NOT NULL, # on users table
So in my design above, you can see I'm really just using the [conversations] table as a placeholder and as a way of grouping messages to a single conversation_id, and then [users_x_conversations] is crosswalk (many-to-many) table where I'm actually storing who is a "member of" which conversation.
Is this the right approach to take or is there a better way to relate the tables here? That's Concern #1.
Assumning I'm modeling the problem at the database correctly, then I have the following JPA/entity classes:
#MappedSuperclass
abstract public class BaseEntity {
#Id
#GeneratedValue(strategy = GenerationType.IDENTITY)
private Long id;
// Ctors, getters & setters down here...
}
#Entity(name = 'messages')
#AttributeOverrides({
#AttributeOverride(name = 'id', column=#Column(name='message_id'))
})
public class Message extends BaseEntity {
#OneToOne(fetch = FetchType.EAGER, cascade = [CascadeType.PERSIST, CascadeType.MERGE])
#JoinColumn(name = 'conversation_id', referencedColumnName = 'conversation_id')
#NotNull
#Valid
private Conversation conversation;
#OneToOne(fetch = FetchType.EAGER, cascade = [CascadeType.PERSIST, CascadeType.MERGE])
#JoinColumn(name = 'user_id', referencedColumnName = 'user_id')
#NotNull
#Valid
private User sender;
#Column(name = 'message_text')
#NotEmpty
private String text;
#Column(name = 'message_sent_at')
#NotNull
private Date sentAt;
// Ctors, getters & setters down here...
}
#Entity(name = 'conversations')
#AttributeOverrides({
#AttributeOverride(name = 'id', column=#Column(name='conversation_id'))
})
public class Conversation extends BaseEntity {
#Column(name = 'conversation_created_on')
#NotNull
private Date createdOn;
// Ctors, getters & setters down here...
}
What I'm stuck on now is: how should I model my [users_x_conversations] table at the JPA layer? Should I create something like this:
#Entity(name = 'users_x_conversations')
#AttributeOverrides({
#AttributeOverride(name = 'id', column=#Column(name='users_x_conversations_id'))
})
public class UserConversations extends BaseEntity {
#ManyToMany(fetch = FetchType.EAGER, cascade = [CascadeType.PERSIST, CascadeType.MERGE])
#JoinTable(
name="users_x_conversations",
joinColumns=[
#JoinColumn(name="user_id")
],
inverseJoinColumns=[
#JoinColumn(name="conversation_id")
]
)
private Map<User,Conversation> userConversations;
// Ctors, getters & setters down here...
}
Basically my service will want to be able to do queries like:
Given a conversationId, who are all the users that are members of that conversation?; and
Given a userId, what are all the conversations that user is a member of (DM and Group Chat alike)?
Is this the right approach to take or is there a better way to relate the tables here?
Your approach seems OK at the DB layer, except that if users_x_conversations serves only as a join table (i.e. if there are no extra properties associated with the (user, conversation) associations represented within), then I would use (conversation_id, user_id) as its PK instead of giving it a surrogate key. If you don't do that, then you should at least put a uniqueness constraint on that pair.
What I'm stuck on now is: how should I model my [users_x_conversations] table at the JPA layer?
I take you to be asking whether you should model that table as an entity. If you insist on giving it a surrogate key as you have done, then that implies "yes". But as I already discussed, I don't think that's needful. Nor much useful, for that matter. I would recommend instead modeling a direct many-to-many relationship between Conversation and User entities, with this table (less its id column) serving as the join table:
#Entity
#Table(name = "converations")
public class Conversation extends BaseEntity {
#Column(name = 'conversation_created_on')
#NotNull
private Date createdOn;
#ManyToMany(mappedBy = "conversations")
#JoinTable(name = "users_x_conversations",
joinColumns = #JoinColumn(name="conversation_id", nullable = false, updateable = false),
inverseJoinColumns = #JoinColumn(name = "user_id", nullable = false, updateable = false)
)
private Set<User> users;
// Ctors, getters & setters down here...
}
#Entity
#Table(name = "users")
public class User extends BaseEntity {
#NotNull
private String username;
#ManyToMany(mappedBy = "users")
// this is the non-owning side of the relationship; the join table mapping
// is declared on the other side
private Set<Conversation> conversations;
// Ctors, getters & setters down here...
}
Note in that case that User and Conversation entities are directly associated in the object model.
On the other hand, if you did choose to model users_x_conversations via an entity of its own, then the code you present for it is all wrong. It would look more like this:
#Entity
#Table(name = "users_x_converations", uniqueConstraints =
#UniqueConstraint(columnNames={"converation_id", "user_id"}))
public class UserConversation extends BaseEntity {
#ManyToOne(optional = false)
#JoinColumn(name = "conversation_id", nullable = false, updatable = false)
Conversation conversation;
#ManyToOne(optional = false)
#JoinColumn(name = "user_id", nullable = false, updatable = false)
User user;
// Ctors, getters & setters down here...
}
Note well that:
This makes the object-level association between Conversations and Users indirect, via UserConversation entities. If the relationships are navigable from the other side, then they would be modelled via #OneToMany relationship fields of type Set<UserConversation> or List<UserConversation>.
It requires more code, and more objects in the system at runtime.
On the other hand, it does have the minor advantage of saving you from making a somewhat arbitrary choice of which side of a direct #ManyToMany relationship is the owning side.
I am having following problem. I have a user entity that has a many to many relationship with other user entities. Hence I want to make a self-join with manytomany annotation. This relation is based on already existing table that is used across the system so I cannot make changes to the DB at all. So we have 2 tables User(Id, ShortName) and UserLink(ParentId, ChildId).
The annotation of ID is assigned to ShortName, but the actual keys in both User and UserLink are ID from User and ParentId and ChildId from UserLink.
I am trying to handle this the following way from the User entity:
#Id
#Column(name = "ShortName")
private String shortName;
#Column(name = "Id")
private long id;
#ManyToMany(cascade = CascadeType.ALL, fetch = FetchType.EAGER)
#JoinTable(name = "UserLink",
joinColumns = { #JoinColumn(name = "ParentId", referencedColumnName = "Id") },
inverseJoinColumns = { #JoinColumn(name = "ChildId", referencedColumnName = "Id") })
private Collection<UserEntity> children;
Since the key in the User entity is on the ShortName field, I have to specify the "Id" as referenced column name param. If I don't do that, it takes the ShortName as the key and doesn't fetch any data.
When I try to do this the way I showed above, I get the following exception:
Caused by: org.hibernate.MappingException: Duplicate property mapping of **_entity_UserEntity_children found in **.entity.UserEntity
at org.hibernate.mapping.PersistentClass.checkPropertyDuplication(PersistentClass.java:486)
at org.hibernate.mapping.PersistentClass.validate(PersistentClass.java:476)
at org.hibernate.mapping.RootClass.validate(RootClass.java:268)
at org.hibernate.cfg.Configuration.validate(Configuration.java:1287)
at org.hibernate.cfg.Configuration.buildSessionFactory(Configuration.java:1729)
at org.hibernate.ejb.EntityManagerFactoryImpl.<init>(EntityManagerFactoryImpl.java:84)
at org.hibernate.ejb.Ejb3Configuration.buildEntityManagerFactory(Ejb3Configuration.java:904)
... 81 more
Do you have any idea how this could be fixed? One idea is that I could change the #Id in the entity and move it to the Id property that is used for joins, but this would need a lot of effort to rewrite bad existing code.
Anyway, is it possible to make a self-join manytomany on columns that are not keys?
Cheers
Adam
I am building a sample for ManyToMany relationship between: User(1) - ()AccessLevel() - (1)Role
I have implemented 3 classes in Java with hibernate implementation as follow:
Class User
#Entity
#Table(name="user")
public class User {
#Id
#GeneratedValue
#Column(name="USER_ID")
#ManyToMany(fetch = FetchType.EAGER, cascade = CascadeType.ALL)
#JoinTable(name = "access_level", joinColumns = {
#JoinColumn(name = "user_id", nullable = false, updatable = false) },
inverseJoinColumns = { #JoinColumn(name = "role_id", nullable = false, updatable = false) })
private Set<Role> roles = new HashSet<Role>(0);
Class Role
#Entity
#Table(name="role")
public class Role {
#Id
#GeneratedValue
#Column(name="role_id")
private Integer id;
#Column(name="role_name")
private String roleName;
Class AccessLevel
#Entity
#Table(name="access_level")
public class AccessLevel {
#Id
#GeneratedValue
private Integer id;
#Column(name="role_id")
private Integer roleId;
#Column(name="user_id")
private Integer userId;
Problem:
When I am persisting the User bean using merge method then an exception arise:
#Service
public class UserServiceImpl implements UserService {
#PersistenceContext
private EntityManager em;
#Override
#Transactional
public void save(User user) {
em.merge(user);
}
Exception
org.springframework.web.util.NestedServletException: Request process
ing failed; nested exception is org.springframework.dao.DataIntegrityViolationException: Could not execute JDBC batch update; SQL [insert into access_level (user_id, role_id) values (?, ?)]; constraint [null]; nested exception is org.hibernate.exception.ConstraintViolationException: Could not execute JDBC batch update
org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:894)
org.springframework.web.servlet.FrameworkServlet.doPost(FrameworkServlet.java:789)
javax.servlet.http.HttpServlet.service(HttpServlet.java:641)
javax.servlet.http.HttpServlet.service(HttpServlet.java:722)
As you can see hibernate is trying to run this query:
insert into access_level (user_id, role_id) values (?, ?)
From my point of view it seems like hibernate is not generating the primary key for AccessLevel even though I have added the #GeneratedValue to the id attribute.
Note:
I am working on production environment with Postgresql and evelopment environment with HSQL database that creates all schemas from the begining based on the entities description. Both environments generate same issue.
Regards,
Cristian Colorado
Reason:
It seems for ManyToMany relationships you do not need to define a class for the "Joining Table". Therefore if I eliminate AccessLevel entity the logic would work perfectly fine. I explain further:
Explanation:
When I defined the User class I also described the relationship with Role:
#ManyToMany(fetch = FetchType.EAGER, cascade = CascadeType.ALL)
#JoinTable(name = "access_level", joinColumns = {
#JoinColumn(name = "user_id", nullable = false, updatable = false) },
inverseJoinColumns = { #JoinColumn(name = "role_id", nullable = false, updatable = false) })
private Set<Role> roles = new HashSet<Role>(0);
Important thing here is I have told hibernate that User entity will relate to Role entity through a table known as "access_level" and such table will have user_id and role_id columns in order to join previous entities.
So far this is all hibernate needs in order to work the many to many relationship, therefore when mergin it uses that information to create and tun this script:
insert into access_level (user_id, role_id) values (?, ?)
Now, the problem cames when I defined a new entity for AccessLevel:
#Entity
#Table(name="access_level")
public class AccessLevel {
#Id
#GeneratedValue
private Integer id;
#Column(name="role_id")
private Integer roleId;
#Column(name="user_id")
private Integer userId;
Now I am telling hibernate that there is a table "access_level" related to AccessLevel entity and it has 3 columns, the most important would be Id which is primary key.
So I defined "access_level" twice!
Solution:
I eliminated the Entity for access_level table.
I re-write my production script in order to have "access_level" with
user_id/role_id columns only.
Note: It would be good to know how to add a primary key to the joining table without generating issues. An alternative would be adding a composed primary key in database(user_id/role_id) which would be independient from hibernate.
Why do you need a PK column in the join table? There will be a composite PK composed of user_id and role_id. Now, as you have discovered a JoinTable for #ManyToMany will only ever have two columns and at some point you may require additional data about this relationship.
e.g.
user_id
role_id
date_granted
You may then want to use your AccessLevel entity however you replace the #ManyToMany with #OneToMany from User to AccessLevel and optionally from Role > AccessLevel.
The Hibernate docs themselves advise against #ManyToMany:
Do not use exotic association mappings:
Practical test cases for real many-to-many associations are rare. Most
of the time you need additional information stored in the "link
table". In this case, it is much better to use two one-to-many
associations to an intermediate link class. In fact, most associations
are one-to-many and many-to-one. For this reason, you should proceed
cautiously when using any other association style.