In Hibernate, to specify a column for joining association, #JoinColumn annotation in used, for example like this:
#ManyToOne
#JoinColumn(name="address_id")
public Address getAddress() {
return address;
}
In most cases, name of the column is snaked-cased class name plus _id. So it is reasonable to expect from Hibernate to derive it automatically (as it is done, for example, in Django's ORM). But is such behavior implemented somehow?
It is not necessary, JPA follows convention over configuration principle which means there are allways some default values that you can override with annotations.
In case of #JoinColumn, the default column name is generated like this: <field_name>_<id_column_name>
field_name is address in your case, and id_column_name is referring to the related entity's id, which is id. Thus, you get address_id by default.
It is not necessary to have #JoinColumn annotation. You can always override it. If you won't provide it in your code then Hibernate will automatically generate one for you i.e. default name for your column.
Related
The NamingStrategy was already being marked as deprecated in Hibernate 4.2/4.3 (HHH-7079). Starting with Hibernate 5, now it will shipped with two replacements(?) interfaces ImplictNamingStrategy and PhysicalNamingStrategy (HHH-7078) and have finally ditched support for the old NamingStrategy. That's why Spring's upcoming Hibernate5 supported has even removed the configurer namingStrategy() and favor of implicitNamingStrategy() and physicalNamingStrategy(). So far, so good.
Although it is mentioned in some documents (i.e. in the incomplete working-5.0-migration-guide.md) and in (generated) release notes for the upcoming Hibernate 5 (as of today), I've found no concrete example how to use these actually.
Perhaps I've missed something, but neither the JavaDoc nor the issues shows any evidence of the idea both strategy types. Furthermore I've already one strategy based on NamingStrategy: a custom OracleNamingStrategy which is based on the well-known one.
While I'm interested in a proper solution in code (obviously), the actual issue is getting a working migration document and/or the conceptual idea of the restructured naming strategy.
Also related:
ImprovedNamingStrategy no longer working in Hibernate 5
Put below key value pair in your hibernate configuration file
hibernate.implicit_naming_strategy=org.hibernate.boot.model.naming.ImplicitNamingStrategyLegacyHbmImpl
hibernate.physical_naming_strategy=org.hibernate.boot.model.naming.PhysicalNamingStrategyStandardImpl
If you are providing #Table and #Column annotation in your entity classes with names provided with an underscore i.e. user_id i.e. #Column(name="user_id"), it will take the column name as user_id; if you give it as userid then it will change to user_id if you use no strategy or implicit strategy (specifically spring.jpa.hibernate.naming.implicit-strategy=org.hibernate.boot.model.naming.ImplicitNamingStrategyLegacyHbmImpl). So, if you want a strategy where the entity attribute name changes to one with underscore and lowercase letters i.e. something from userId to user_id, you should use implicit or no strategy (which actually uses implicit strategy).
If you don't want your naming strategy to add an underscore to the column name or class name, then the strategy that you need to use would look like:
spring.jpa.hibernate.naming.physical-strategy=org.hibernate.boot.model.naming.PhysicalNamingStrategyStandardImpl. The things that you provide in annotations #Table and #Column’s name attribute would remain as it is.
If you don't want to provide annotations and want to manually handle the table name and column names, you should extend the class org.hibernate.boot.model.naming.PhysicalNamingStrategyStandardImpl and override the required methods. If you still use annotations for some of the cases here, remember the overridden methods will apply on the names written in those annotations.
spring.jpa.hibernate.naming.physical-strategy=example.CustomStrategy
I have an enum attribute inside a backing bean which haves 2 attributes: "id" (int attribute) and "label" (String attribute). In the DB I need to persist only the "id", but I still need the label value to use on my view. The problem is: #Enumerated only gives me the option to persist using EnumType.ORDINAL or EnumType.STRING. There's any way to persist just the enum's "id" attribute in the DB? (btw... i'm using Hibernate).
Thanks!
fd's comment is in place. Nevertheless, if you're in a real need of this you can tweak the solution a bit, instead of persisting an enum, you can mark it as transient and add a property e.g. enumId to an entity class, that you'll persist instead. Than the only thing left is to synchronize the enum's value through getter and setter method of an enumId
The id and the attribute are two facets of the same thing. They have the same meaning. Storing them both in the DB is redundant, and violates the Third Normal Form of database design.
For a JPA (Hibernate) entity, I suggest you define the ORDINAL (which is the id) as your field and as suggested above by "Master Slave" define the String as a transient. Then define #PostLoad and the setter of the id to convert the ordinal to the name (which is your label) using name() and convert back using valueOf(String name) in the setter of the label.
What is the difference between #AttributeOverride and #AssociationOverride. Please explain me with the simple example.
From official documentation:
#AssociationOverride
Used to override a mapping for an entity
relationship
#AttributeOverride
Used to override the mapping of a Basic (whether explicit or default)
property or field or Id property or field.
Read official javadoc: is well explained with some example.
May be that is a bit more simple:
The #AttributeOverride annotation is used to override the mapping of property or field or Id property or field.
The #AssociationOverride annotation is used to override a many-to-one or one-to-one mapping of property or field for an entity relationship.
https://javabeat.net/eclipselink-jpa-annotations-attributeoverride-associationoverride/
What is the difference between #Embedded annotation technique and #OneToOne annotation technique because in Embedded the java class contain "Has a" relationship in class and with the help of #Embedded annotation we persist the has a object in database. and in OneToOne relationship we also persist the has a object in database.
#OneToOne is for mapping two DB tables that are related with a one to one relationship. For example a Customer might always have one record in a Name table.
Alternatively if those name fields are on the Customer table (not in a separate table) then you might want an #embedded. On the face of it you could just add the name fields as standard attributes to the Customer entity but it can be useful if those same columns appear on multiple tables (for example you might have the name columns on a Supplier table).
Its the difference between composition and aggregation. #Embedded objects are always managed within the lifecycle of their parents. If the parent is updated or deleted, they are updated or deleted as well. #OneToOne objects may mimic composition via the cascadeType option of their #Join annotation, but by default they are aggregated, aka their lifecycle is separate from that of their parent objects.
#Embedded is used with Value Objects (Objects which have a meaning only when attached to an Object) whereas one to one mapping is between two objects having their own existence and meaning.
For e.g.
Value Object and #Embedded: If we have a User class and this class has an address Object in it, it can be considered as a value object as the address alone does not have any significance until unless associated with a user. Here address object can be annotated with #Embedded.
One to One mapping and #OneToOne: If we have a User class and this class has a 'Father' Object or a 'Mother' object, we would want to annotate the 'Father' or 'Mother' instance as #OneToOne as 'Father' or 'Mother' have their own meaning and existence and are not Value objects to User class.
A closely related difference is between #OneToMany and #ElementCollection. Both are used to save instance variables of Collection type in Java class. The difference being, #ElementCollection is to be used when the elements of Collection being saved are Value Objects whereas #OneToMany is used when the elments and object have well defined meaning and existence.
Use #OneToOne, only if fields can be reused. Otherwise, go for #Embeddable.
A quote from Beginning Hibernte, 3rd Edition:
There is nothing intrinsically wrong with mapping a one-to-one association between two entities where one is not
a component of (i.e., embedded into) the other. The relationship is often somewhat suspect, however. You should
give some thought to using the embedded technique described previously before using the #OneToOne annotation.
#Embeddable:
If the fields in an entity (X) are contained within the same table as another entity (Y), then entity X is called "component" in hibernate terms or "embedded" in JPA terms. In any case, JPA or hibernate do not allow to use 2nd table to store such embedded entities.
Generally, we think of normalizing a table when data is being reused by more than one table. Example: A Customer (id, name, street, city, pin, landmark) can be normalized into Customer(id, name) and CustomerAddress(cust_id, street, city, pin, landmark). In this case, we can reuse CustomerAddress by linking the same using cust_id with other tables. But if this reuse is not required in your application, then we can just keep all columns in one table.
So, a thumb rule is,
If reuse -> #OneToOne,
If no reuse -> #Embeddable
#Embedded is typically to represent a composite primary key as an embeddable class:
#Entity
public class Project {
#EmbeddedId ProjectId id;
:
}
#Embeddable
Class ProjectId {
int departmentId;
long projectId;
}
The primary key fields are defined in an embeddable class. The entity contains a single primary key field that is annotated with #EmbeddedId and contains an instance of that embeddable class. When using this form a separate ID class is not defined because the embeddable class itself can represent complete primary key values.
#OneToOne is for mapping two DB tables that are related with a one to one relationship. #Id will be the primary key.
Using JPA, can we define an enum as id of an entity?
I've tried the following:
public enum AssetType {
....
}
#Entity
#IdClass(AssetType.class)
public class Adkeys {
private AssetType type;
#Id
#Enumerated(EnumType.STRING)
#Column(nullable = false)
public AssetType getType() {
return type;
}
}
Using OpenJPA, it complains:
org.apache.openjpa.persistence.ArgumentException: The id class "class aa.AssetType" specified by type "class aa.Adkeys" does not have a public no-args constructor.
So my questions are:
should we able to use enum as id for an entity on JPA? (i.e. there is a bug in OpenJPA)
or do I make a mistake somewhere?
and is there any workaround for such problem?
The JPA spec doesn't say this is possible:
2.1.4 Primary Keys and Entity Identity
The primary key (or field or property of a composite primary key) should be one of the following types: any Java primitive type; any primitive wrapper type; java.lang.String; java.util.Date; java.sql.Date. In general, however, approximate numeric types (e.g., floating point types) should never be used in primary keys. Entities whose primary keys use types other than these will not be portable.
If you really want to have a compile-time fixed number of records for a given entity, you can use a String or int primary key and assign it AssetType.FOO.name() or AssetType.FOO.ordinal()
And non-portable here means that some persistence provider may support other things, but it might not work for another provider. As with the enum - if the persistence provider has special support for it, that does not try to instantiate it, but rather processes it specially after checking if class.isEnum(), then it might work. But it seems your persistence provider doesn't do this.
No, you can't use enums as ID because JPA doesn't allow to define your own mapping for ID columns (they must be int or long or something that JPA can create with new).
IDs must not be the business key (in your case: the type). Using the business key as an ID is a common mistake in DB designs and should be avoided because it will cause all kinds of problems later.
Add an independent ID column to solve the problem.
OpenJPA is the only JPA provider that does not support this.
See Support Enum as Primary Key Type
Do you really want to do this? This construct doesn't allow changing the database enum keys without updating the enum in the code (fail on load), nor the other way around (constraint failure). Why don't you just create an AssetType table with int pk and name, and make the Adkeys have a foreign key to AssetType.id as pk?
You can load the AssetTypes from the db on startup if you need to enumerate them in your app.