- Hibernate Manytomany The Join Table Has Sequence Generated Primary Keyboard
- Hibernate Manytomany The Join Table Has Sequence Generated Primary Keys
Every JPA entity is required to have a field which maps to primary key of the database table. Such field must be annotated with @Id.
Jun 23, 2009 Re: JPA, Hibernate: Custom primary key generator, before persist 843859 Jun 22, 2009 2:34 PM ( in response to Tolls ) So, you want to generate the barcode on the document, but ensure that the code is unique (ie using the database to keep track of the id possibly). Consider the following model of the relationships between Work, Author and Person.We represent the relationship between Work and Author as a many-to-many association. We choose to represent the relationship between Author and Person as one-to-one association. Command to generate ssh key in hadoop. Another possibility would be to have Author extend Person.
Simple vs Composite primary keys
A simple primary key consists of a single Java field which maps to a single table column.
A composite primary key consists of multiple Java fields which individually map to separate columns.
Supported types for a primary key
A simple primary key field or one of the composite primary key field should be one of the following types:
- Any Java primitive type
- any Any primitive wrapper type
- java.lang.String
- java.util.Date
- java.sql.Date
- java.math.BigDecimal
- java.math.BigInteger
In this tutorial we are going to focus on generation strategies of simple primary key.
@GeneratedValue Annotation
This annotation defines the types of primary key generation strategies. If this annotation is not used then application is responsible to populate and manage @Id field values itself.
The use of the GeneratedValue annotation is only required to be supported for simple primary keys.
GenerationType enum defines four strategies: Generation Type . TABLE, Generation Type. SEQUENCE, Generation Type. IDENTITY and Generation Type. AUTO. Let's understand them with examples. Wep key generator.
GenerationType.SEQUENCE
With this strategy, underlying persistence provider must use a database sequence to get the next unique primary key for the entities.
We have created the following Util class to reuse the code for other examples.
Also, in the persistence.xml, we have created four persistence-unit, so that we can try four GenerationType independently. We are using Hibernate as persistence provider.
Let's create main class to try out Entity1 key generation.
Output
Above output shows one table MYENTITY1 and one sequence HIBERNATE_SEQUENCE are created.
GenerationType.TABLE
With this strategy, underlying persistence provider must use a database table to generate/keep the next unique primary key for the entities.
Output
This time no sequence is generated, instead an additional table named 'HIBERNATE_SEQUENCES' is created to maintain primary key sequence.
GenerationType.IDENTITY
This GenerationType indicates that the persistence provider must assign primary keys for the entity using a database identity column. IDENTITY column is typically used in SQL Server. This special type column is populated internally by the table itself without using a separate sequence. If underlying database doesn't support IDENTITY column or some similar variant then the persistence provider can choose an alternative appropriate strategy. In this examples we are using H2 database which doesn't support IDENTITY column.
Output
Above output shows that a sequence is used for primary keys.
GenerationType.AUTO
This GenerationType indicates that the persistence provider should automatically pick an appropriate strategy for the particular database. This is the default GenerationType, i.e. if we just use @GeneratedValue annotation then this value of GenerationType will be used.
Hibernate Manytomany The Join Table Has Sequence Generated Primary Keyboard
Output
Above output shows that a sequence is used for primary keys.
When @GeneratedValue not used
If we don't use @GeneratedValue annotation at all, then we have to populate the unique primary keys ourselves. In this example, we are simply assigning it to the value returned from System.nanoTime()
Output
Above output shows that a no sequence or extra table were generated.
Example Project
Dependencies and Technologies Used:
- h2 1.4.193: H2 Database Engine.
- hibernate-core 5.2.8.Final: The core O/RM functionality as provided by Hibernate.
Implements javax.persistence:javax.persistence-api version 2.1 - JDK 1.8
- Maven 3.3.9
Hibernate Manytomany The Join Table Has Sequence Generated Primary Keys
Having said that, it is better to use a surrogate key for the primary key to protect against business semantics changes in the natural key. So why not use a generated id?
However, if you cannot change the data model, you can map the entire table as one big composite key. A horrible solution, but a work around which gives you time to fire the DBA who modelled your data this way, and hire a competent replacement.
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Originally posted by Ehrenfrids:
I have a table which has no primary key. How to create the hibernate mapping xml without using the generator id
How are you accessing records in that table when not using Hibernate? i.e. when you use SQL directly.
So: I can't add surogate key (I can't change the database) and I don't have a natural (business) key.
What about that?
Originally posted by Ionescu Victor:
The problem might not be so simple to adress. Suppose I work with a legacy database and I have a readonly table (imported from a back-office) and I don't want to access individual rows in that table (I want just to list the records obtained form a join).
So: I can't add surogate key (I can't change the database) and I don't have a natural (business) key.
What about that?
See my earlier answer. That's the only work around you have available to you.
I'm no expert on Hibernate but this should be easy
An Object-Relational Mapping tool can only be expected to map relational data. Data without a primary key it is not relational, so it should be no surprise it doesn't work.
You could use a named query to insert data instead of declaring an entity, and perform the insert from the dao.