I have a database that, there is relations between some of tables of it,
for example, I have to table Person and Company, and there is a one to many relation between their Id that is build in a table name Person_Company generated by hibernate.
when I use #OnetoMany and #ManytoOne annotations in java,
It make relation between them, but not according to Person_Company table
and I use just Id 's of table
what is correct annotation?
You want the association to be mapped using a join table. So the annotation is (drumroll...) #JoinTable. The javadoc has examples, and the Hibernate documentation as well.
Related
here I'am, trying to get a better understanding of JPA Relationships in SpringBoot.
Those Entities has been created following some XML configuration files.
I'm not used to XML and I'm trying to improve my Java/SpringBoot #Annotation skills.
I'll show you 3 Entities with a OneToOne & ManyToOne annotations.
Questions
In Driver -< ServiceRequestDriverVehicle (#OneToMany) the mappedBy="driverId" references the current Driver id field or the JoinColumn(name="driverId") from the other Entity? e.g.: In Vehicle Entity mappedBy use vehicle, which if comes from ServiceRequestDriverVehicle it's named as vehicleId #JoinColumn(name="vehicleId") but it's field name is vehicle, so why where is using vehicle instead of vehicleId as in the Driver case does? Again this is abstracted from an XML configuration that can be wrong.
Are the arrows from the images pointing the right meaning of the Annotation? I mean, is the annotation taking the entity/class of the annotated field to make the relationship mapping?
In the #OneToOne relationship(Driver), is the referencedColumnName="ID" mandatory/necessary or OneToOne is automatically linked to the PK (id)
I have a MySQL model where the Set fields do not appear, should I Annotate those as #Transient or the LAZY fetch is enough?
Thank you for your answers, I hope this can solve future misunderstandings
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.
As touched on in the following:
Cascade on delete using unidirectional Many-To-Many mapping
How to remove entity with ManyToMany relationship in JPA (and corresponding join table rows)?
JPA JPQL: select items when attribute of item (list/set) contains another item
when removing entries from #ManyToMany mappings, there is an element of manual tweaking involved to remove the foreign keys.
I have been using my own JPA CRUD for years and have a nice solution for this: CrudDao e.g. NoteDao.
But I've been recently very impressed by Spring JPA and the ability to autogenerate Cruds by declaring an interface and then creating similarly named namedQuerys on the #Entity.
But how do I fix the #ManyToMany remove problem? I'll essentially have remove methods on my Crud that expose this "feature" of JPA.
I appear to have solved this using #PreRemove, as noted in https://stackoverflow.com/a/14911910/1041691
e.g.
https://github.com/fommil/zibaldone/blob/master/src/main/java/com/github/fommil/zibaldone/Note.java#L74
I have two tables named USER and ROLE with entities as USER and ROLE resply.
Now I want to create one more entity by using these above entity so that I can fetch datas of both tables.
One more table is there named USER_ROLE which contain two fields "user_id" and "role_id".
I have to accomplish these thing using hibernate annotations.
I tried by creating third entity with annotated table as USER table and properties are some USER table fields and Collection set of ROLE table. But here's the mistake is I annotated the third entity with USER table which is already annotated with User entity.
So now I have to make that third entity without annotating any table, has to access the two entities which are already accessing the table .
how can i accomplish these using hibernate annotations.
kindly help me on this issue
thanks, rajNaveen
As far as I understand what are you looking for can be found in this tutorial.
Just see how #JoinTable is used.
Hope that helps !!!
I am thinking about switching from a self-implemented versioning-solution to Hibernate Envers, but I am not quite sure yet. I have read a lot about it, but I am concerned about schema changes and how Envers deals with them after having historized data according to an older schema.
What is your experience with Envers in this regard? How do you deal with schema changes and existing data with Envers?
Update 1:
It is not just about adding removing simple columns from a table, but e.g. when changing a simple Forein-Key-Relationship into a separate entity with two 1:n-relationships (M2M with attributed columns. This is a "logical" change in your data model. How do you deal with that when using Envers, when there is already historized data according to the old model? Is there an alternative to manually write sql-scripts and transfering them into the new representation?
In my experience, Envers simply copies every field from your entity table to its audit tables. The copied fields in the audit tables have no constraints on them, including nullability and foreign key constraints, so there's no problem with adding or removing such constraints on the real tables. Any kind of relationships you add to your entities will just be new audit columns and/or tables added under Envers, and it's up to you to correctly interpret them in their historical context.
For your example, if I understand correctly, of switching from a join-column-based relationship to a join-table-based one, you'd simply have the old join column coexisting with the join table, and at the point of the cutover, the former will cease being populated in favor of the latter. Your history will be completely preserved, including the fact that you made this switch. If you want all the old data to fit into the new model in the audit tables, it's up to you to do the migration.
There shouldn't be problems with modifying the existing schema as Envers relies on your #Entities to create the audit tables. So if you add or remove a column from an existing table, as long as this change is reflected in your #Entity / #Audited JavaBean, it should be ok.
The foreign key refactoring should be fine with Envers. As Envers creates a join table even for one-to-many relationship, it should be straight to change it to become many-to-many relationship. I extracted one paragraph from official document:
9.3. #OneToMany+#JoinColumn
When a collection is mapped using these two annotations, Hibernate
doesn't generate a join table. Envers, however, has to do this, so
that when you read the revisions in which the related entity has
changed, you don't get false results.
To be able to name the additional join table, there is a special
annotation: #AuditJoinTable, which has similar semantics to JPA's
#JoinTable.
One special case are relations mapped with #OneToMany+#JoinColumn on
the one side, and #ManyToOne+#JoinColumn(insertable=false,
updatable=false) on the many side. Such relations are in fact
bidirectional, but the owning side is the collection (see alse here).
To properly audit such relations with Envers, you can use the
#AuditMappedBy annotation. It enables you to specify the reverse
property (using the mappedBy element). In case of indexed collections,
the index column must also be mapped in the referenced entity (using
#Column(insertable=false, updatable=false), and specified using
positionMappedBy. This annotation will affect only the way Envers
works. Please note that the annotation is experimental and may change
in the future.