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.
Related
I'm trying to save a model with libraries that are stored in cascade. In the model we use the following notation for it:
#ManyToOne(fetch = FetchType.LAZY)
#JoinColumn(name = "name_table_column")
#Cascade (CascadeType.SAVE_UPDATE)
This means that when saving the model, insert or update these libraries according the libraries exist or not. In this case we just wanted to perform the inserts and ignore all the updates. Is there any way for it?
Thanks!
I guess you are looking for CascadeType.PERSIST:
CascadeType.PERSIST: cascades the persist (create) operation to
associated entities persist() is called or if the entity is managed
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.
I've been using JPA 2.0 for a while but, sad to admit, I haven't had enough time to learn it properly. It seems like I lack the basics of how to work with Entity Manager.
Moving one step at a time, I'd like to first ask you about maintaining relationships between mapped entities. Of course I know how to create mappings between entities, different types of available associations (OneToOne, etc.) and how databases work in general. I'm purely focused on maintaining it via Entity Manager, so please do not send me to any kind of general knowledge tutorial :-).
The questions are:
Am I right that as a programmer I'm responsible for maintaining (creating/updating/removing) relationships between instances of entities?
Do I have to always update (set to null, remove from collection, etc.) instances by hand?
Plain SQL can set entities to NULL on deleting, but it seems like JPA can't do such a simple thing. It also seems like a burden to do it manually. Is there a way to achieve that with JPA?
If I have OneToMany relationship and set to NULL the entity on the Many side of the relationship. Then I persist the changes in a Set by saving the entity on the One side. Do I then have to update the entities in the Many side and set association to NULL in each instance? Seems pure silliness for one-directional bindings!
Thanks in advance!
The main thing you need to investigate is the different options you have when mapping on entity. For example in the next piece of code the cascade all option will instruct jpa to delete the child list when the parent is deleted.
#OneToMany(fetch = FetchType.LAZY, cascade = { CascadeType.ALL }, mappedBy = "parent")
private Set<Child> events = new HashSet<Child>();
Yes. You maintain the object tree and modify it to look like what
you want.
Yes and no. If you want the entity to reference null, then yes.
For instance, if you are removing one Entity, then you should clean
up any references to it held by other entities that you are not
removing. A practical example: its good practice to let an Employee
know his/her Manager has been let go. If the Employee is going to
stay, it should either have its manager reference nulled out or set
to a different manager, before the current manager can be removed.
If the employee is going to be removed as well, then cascade remove
can cascade to all the Manager's subordinates, in which case you do
not need to clean up their references to the manager - as they are
going away too.
I don't quite understand what SQL is setting to null. Deleting
removes the row in the database, so there isn't anything to set to
null. Cleaning up a reference shouldn't be that difficult in the
object model, as JPA has a number of events to help such as
preremove preupdate etc. In the end though, the problem is with
your java objects. They are just java objects, so if you want
something done, your application will need to do it for the most
part. JPA handles building them and pushing them to the database,
not changing the state for you.
Yes and no. If you set up a bidirectional relationship, you must
maintain both sides as mentioned above. If you set the child's
parent reference to null, you should let the parent know it no
longer has a child, wouldn't you? Your parent will continue to
reference its child for as long as that Parent instance exists. So
even though the database is updated/controlled through the side that
owns a relationship, the object model will be out of synch with the
database until it is refreshed or somehow reloaded. JPA allows for
multiple levels of caching, so it all depends on your provider setup
how long that Parent instance will exist referencing a Child that no
longer exists in the database.
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 a many-to-many relationship were the link table has an additional field. Hence the relationship is done with 2 one-to-many relations according to below tutorial:
http://www.mkyong.com/hibernate/hibernate-many-to-many-example-join-table-extra-column-annotation/comment-page-1/#comment-122181
I have the 2 entities, a third entity which defined the link table and consist of an #Embeddable ID field.
The relationship is defined as:
#OneToMany(fetch = FetchType.LAZY, mappedBy = "pk.compound", cascade = CascadeType.ALL)
#OneToMany(fetch = FetchType.LAZY, mappedBy = "pk.structure", cascade = CascadeType.ALL)
were pk = the #Embeddable ID field. Insert and deleting works fine however when I call
session.merge(compound);
I get a StackOverflowError and log shows that hibernate is making tons of select statements. Note that database contains exactly 1 association, eg. 1 compound containing 2 structures. It looks like hibernate gets into a endless loop.
I've seen this solution also on http://giannigar.wordpress.com/2009/09/04/mapping-a-many-to-many-join-table-with-extra-column-using-jpa/ but how do you do updated with this?
My solution was to use FetctType.EAGER on the owning side and FetchType.Lazy on child side and in the 2 ManyToOne relations in the link table. Like this I can load from the owning side without getting LazyInitializationException and mergign working as expected.
I second SpaceTuckker's answer. I don't think Persist is the same as merge. Merge will load the object before persisting it. Persist doesn't. So when you call merge IMHO it needs to load the lazy relation. If you call persist it does not.
You might also use #ElementDependent to mark related #OneToMany relations as depending on another table. This was the way I solved the Many-To-Many relation with additional columns in the join table.