I have a Task entity that should contain an array of User entities in a #OneToMany relationship.
But I can't figure out a way to store said array without having a link table, since a User may be referenced in multiple Tasks.
The relationship should look like such, in TaskEntity:
#OneToMany(mappedBy = "user")
private Set<UserEntity> users = new HashSet<>(0);
What would be the proper way of doing it? How can a single row save an array of entities?
You can avoid join table (but it is not advised to do so, see below) for unidirectional one-to-many relationship. It would require foreign key to be on the target side of relationship(many side of relationship). You would need to specify a #JoinColumn annotation to point to the foreign key column.
#OneToMany
#JoinColumn(name="TASK_ID")
private Set<UserEntity> users = new HashSet<>(0);
It is not advisable, because of the following :
1) Performance : if both UserEntity state and Task states are changed, upon writing to UserEntity the FK to Task is not known, because UserEntity does not have reference to it. So in this case UserEntity might be written twice, once for UserEntity changes and once for Task changes.
2) Mapping : if UserEntity is assigned to a different task, and there is no reference back to Task from UserEntity, there will be no changes in context.
So it is advisable to go for Join Table in your case.
So a User can have many Tasks and each Task can have many Users.
Sounds like a classic fan-trap, you will need a link table and to use #ManyToMany.
However the Data-Structure should be modeled before looking into data access implementation.
What you have described is a Many to Many relationship, and your database structure should reflect that, before you think about how you can retrieve the data with Hibernate.
Related
After some frustrating issues and tests, I've read that hibernate can't lazily fetch ToOne relationships.
From what I've read, hibernate lazily fetches ToMany by setting its own Set as a proxy, and when a method is called on that Set, it fetched the data in the database before performing the action. Fine.
For ToOne, the reason I've seen is that since the attribute can be null (unlike ToMany), hibernate has to know whether it needs to populate it with null or a proxy, and that hibernate cannot know that without querying the other table. And since it has to query that other table, it eagerly fetches the data at the same time.
I find that rather stupid. I can understand it on the non owning side of the relationship, where nothing in the table indicates whether the toOne is populated, but on the owning side, the table contains a column with the foreign key, which is either null or not.
Why can't hibernate query the table and set the attribute to either null or a proxy depending on the value from that column? It doesn't need to check the second table, if your column is not null, you know the second table has a corresponding entry (and if it hasn't, you have an integrity problem and hibernate should just throw).
Hibernate behaves more or les how you descibed.
On the owning side hibernate supports Lazy loading, it's just not enabled by default. You need to add it #OneToOne(fetch = FetchType.LAZY)
But when you have that mapped bidirectionaly (on both entities), as you said hibernate needs to query the table to decide between null or proxy. So devs decided to eager load the whole entity. Regardless of the fetch type.
You can avoid theese problems by getting rid of the foreign key and just use same primary key vaue.
You can do that with #MapsId annotation on the owning side.
#Entity
public class Owning {
#Id
private Long id;
#OneToOne
#MapsId
#JoinColumn(name = "id")
private Child child;
}
Recently, I have been learning about Hibernate, and I am facing some difficulties. My first problem is as follows: I am very much confused with the below terms.
Bidirectional mapping
Many to One
Because, as far as I know, in rdbms we first need to insert in parent table. Then we can insert on child table, so the only possible scenario is one-to-many (first parent then children). Then, how is many-to-one is going to work? Second, what is this bidirectional mapping in regards to Hibernate. Specifically, different types of join annotations confuse me a lot. I am listing those annotations below.
1.#JoinTable(name = "Tbale_Name", joinColumns = { #JoinColumn(name = "Column_Name") },
inverseJoinColumns = { #JoinColumn(name = "Another_ColumnName") })
2.#OneToMany(mappedBy="department")` this mappedby term
3.#PrimaryKeyJoinColumn
Please help me understand these concepts.
The first thing I would say is don't think in terms of tables but think in terms of Objects.
What you are trying to express with the annotations is the relationship between objects, let hibernate work out how to persist the data. You can obviously manually check the SQL but the idea of using an ORM is to map the relationships between entities accordingly and let the ORM figure out the complexity around generating SQL etc.
Its worth noting that the parent -> child relation can be mapped using #ManyToOne by adding mappedBy to the non-owning (child) side of the relationship. Hibernate then will determine which entities to insert into the database first. Running with a TransactionManager will enforce integrity with multi table inserts. Hibernate will also workout which entities need to be persisted, for example if you add an new object on the many side to an existing object on the one side.
Furthermore, its worth understanding that in some cases it won't always be the database that generates the primary key in a parent -> child foreign key. Its possible for the code to generate the Identifier and hibernate will persist them according.
Bidirectional mapping means that object entities have a reference to each other. i.e. You can retrieve the second entity from the first entity. Bidirectional mapping supports one-to-many
or many-to-many. I.e. OneToMany = a Set on one of the entities. Many-To-Many = Sets on both entities.
JoinTable tells hibernate that a table in the database can be used to map to other tables together. See JPA "#JoinTable" annotation for more information. JoinColumn tells hibernate what column to use to make the join between the two entities. Hibernate needs these to construct the SQL.
I have the following #OneToOne relationship:
#Entity
public class CarUser {
#OneToOne
#JoinColumn(name = "use")
private User user;
}
#Entity
public class User {
}
Basically, the User is in the core model and CarUser is in an extension model. And User shouldn't know anything about CarUser (I cannot define an inverse relation on it).
The question is When I delete the User, is there anyway I can cascade delete the CarUser as well?
By definition, if you want to state that "User shouldn't know anything about CarUser" you can't get the persistence layer to cascade for you.
You don't necessarily need to make that statement, though - it may not be a correct design understanding. It's reasonable for entities, within the same database schema, to know about each other.
Let's put it this way -- even though (in an manufacturing/accounting system) CostingModule and LedgerModule are in different modules, they absolutely are expected to communicate and interact.
You can also do it with a foreign-key constraint in the database instead, or with triggers.
alter table CARUSER add constraint CARUSER_USER foreign key (ID)
references "USER" (ID) on delete cascade;
I've been trying to recreate the parent/child scenario with hibernate, I used this as an example (very new to hibernate):
http://www.mkyong.com/hibernate/hibernate-one-to-many-relationship-example-annotation/
My situation is similar to this. I am working on a project that has some task objects which should be part of some task flow object.
So there are two entities task and taskflow.
Taskflow should have many or at least one task.
There are already some tasks in the task table, now I want to insert some entries into taskflow table so that taskflow object has list or set of tasks.
Forgot to mention in my original question :
Task can be inside many different task flows
My taskflow looks something like :
class Taskflow{
int taskflowid;
String taskflowname;
List<Task> tasklist;
}
in the database taskflow has 2 fields taskflowid and taskflowname.
This is how Task looks like :
class Task{
//many properties/fields
}
Taking this example into consideration when compared to examples I googled, there is always some kind of relation from Task back to the TaskFlow. Is that necessary? I just want taskflow to have reference to task. Am I thinking this one all wrong?
If you don't declare relationship it would be simple member variables. In order to hibernate understand that you have one to many relationship you need to specify it in some for .hbm , annotation
#OneToMany //This for hibernate to understand that you have one to many relationship with Task
List<Task> tasklist;
inside Task
#ManyToOne
Taskflow taskFlow
Remember default names will be used to generate column names. Please refer tutorial over here
Hibernate supports relationships of different cardinality (one-to-one, one-to-many/many-to-one, many-to-many) and different directionality (unidirectional or bidirectional).
In your case you have a many-to-many relationship, which can be either unidirectional (if Task don't need to contain a collection of all TaskFlows it's a part of), or bidirectional.
For example, a unidirectional relatioship from TaskFlow to Task:
class Taskflow {
...
#ManyToMany
private List<Task> tasks;
...
}
See also:
Chapter 7. Collection mapping
My current project uses JPA and HSQLDB.
I would like to persist multiple related objcts at one go, is that by any means possible in JPA?
Ex: Suppose there are two entities like Person and ContactInfo, where Person has List<ContactInfo> entities.
If I want to persist Person entity along with ContactInfos also, what I am doing is set the list in Person and call persist. Will doing that take care of persisting List<ContactInfo> also? (With foreign key reference to Person ID in database table)
Else kindly let mek now how would I achieve this in JPA.
Regards,
Satya
It will, if you set #*ToMany(cascade=CascadeType.PERSIST)
You could do as Bozho suggested, but if you would also like them to be updated, deleted, etc. when it's done with Person, I would suggest to cascade like that:
#OneToMany(cascade = CascadeType.ALL, orphanRemoval = true)
Note: orphanRemoval will only work with JPA 2.