Id Primary Key Generator Nhibernate

The most common primary key mapping is available through Id method. There are several features missing - like unsaved-value (will be added in NHibernate 3.3) or DDL column options. Probably the most important option is Generator. It specifies the way NHibernate (or database) generates unique values.

5 Aug 2012CPOL
  1. NHibernate Avoid identity generator when possible. Time to read 1 min 161 words. Avoid identity generator when possible (26 Mar 2007) Nullable DateTime Issues. The point being that ID:s used for relational primary keys are a navigational persistence concern rather than a concern of the Entity.
  2. Hello, I'm working with JPA and Hibernate, and project requirement is to have primary key (id) value in application, before entity is persisted. JPA and Hibernate have techniques for generating id's during persist, but I need those id's before, and I need them to be unique.
This example demonstrates how to work on mappings with associations for legacy database tables designed purely using multiple primary keys/composite keys.

Introduction

When I was integrating NHibernate into an existing enterprise software with a legacy database, it took me some time to figure out how mappings for composite keys with associations between entities should be set. This simplified example might be useful to start with for who's trying the same thing.

Background

The article NHibernate and Composite Keys by Anne Epstein provides a thorough explanation on fundamental composite keys mapping and supporting lazy loading by declaring the composite keys as a class object. Here we only focus on a simple code sample of the associations between entities with composite keys.

Using the Code

Considering 2 sample tables, Products and Orders:

Products

NameType
StoreIDNUMBERPK
ProductIDNUMBERPK
ProductNameVARCHAR

Orders

NameType
StoreIDNUMBERPK
ProductIDNUMBERPK
OrderIDNUMBERPK
AmountNUMBER

There is no relationship between both tables in the database but apparently the association from Products to Orders is one-to-many.

We define a ProductIdentifier class as the ID of the Product entity class which contains the primary keys, StoreID and ProductID.

In the Product entity class definition, we add a ISetcollection property of Order entity type. The collection property represents the Order class objects associated with the Product class object. We need to include the Iesi.Collectionlibrary which comes with the NHibernate package.

In the Product XML mapping file the ID of Product entity is mapped by <composite-id> element.

The ISetcollection property defined previously is mapped by <set> element:

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  • The <key> elements here is like the foreign key of the Orders table, in this case they are StoreID and ProductID.
  • The <one-to-many> element specifies the association.
  • Setting the inverse attribute to true specifies that the collection is a mirror image of the many-to-one association in the Order entity.

The mapping of Orders table is similar to what we just did above. Since it has a many-to-one association with Products, we add a Product class type property in the Order class definition.

In the corresponding mapping file, the <many-to-one> element specifies the association with Product entity and the <column> elements are the foreign keys of the Orders table referring to the Products table.

That鈥檚 all needed for the association to work. Adding a <version> element in the mapping is recommended for long transactions if adding a new version column of the table in the legacy database is allowed.

History

  • 2012-8-6 Corrected <many-to-one> element description
  • 2012-7-12 Submitted

The <generator> class is a sub-element of id. It is used to generate the unique identifier for the objects of persistent class. There are many generator classes defined in the Hibernate Framework.


All the generator classes implements the org.hibernate.id.IdentifierGenerator interface. The application programmer may create one's own generator classes by implementing the IdentifierGenerator interface. Hibernate framework provides many built-in generator classes:

  1. assigned
  2. increment
  3. sequence
  4. hilo
  5. native
  6. identity
  7. seqhilo
  8. uuid
  9. guid
  10. select
  11. foreign
  12. sequence-identity

1) assigned

It is the default generator strategy if there is no <generator> element . In this case, application assigns the id. For example:

2) increment

It generates the unique id only if no other process is inserting data into this table. It generates short, int or long type identifier. If a table contains an identifier then the application considers its maximum value else the application consider that the first generated identifier is 1. For each attribute value, the hibernate increment the identifier by 1. Syntax:

3) sequence

It uses the sequence of the database. if there is no sequence defined, it creates a sequence automatically e.g. in case of Oracle database, it creates a sequence named HIBERNATE_SEQUENCE. In case of Oracle, DB2, SAP DB, Postgre SQL or McKoi, it uses sequence but it uses generator in interbase. Syntax:

For defining your own sequence, use the param subelement of generator.

Foreign Key

4) hilo

Primary Key Adalah

It uses high and low algorithm to generate the id of type short, int and long. Syntax:

5) native

It uses identity, sequence or hilo depending on the database vendor. Syntax:

6) identity

Asa 5505 license key generator reddit. It is used in Sybase, My SQL, MS SQL Server, DB2 and HypersonicSQL to support the id column. The returned id is of type short, int or long. It is responsibility of database to generate unique identifier.

7) seqhilo

It uses high and low algorithm on the specified sequence name. The returned id is of type short, int or long.

Pengertian Primary Key

8) uuid

It uses 128-bit UUID algorithm to generate the id. The returned id is of type String, unique within a network (because IP is used). The UUID is represented in hexadecimal digits, 32 in length.

9) guid

It uses GUID generated by database of type string. It works on MS SQL Server and MySQL.

10) select

It uses the primary key returned by the database trigger.

11) foreign

It uses the id of another associated object, mostly used with <one-to-one> association.

Id Primary Key Generator Nhibernate 2017

12) sequence-identity

Id Primary Key Generator Nhibernate Test

It uses a special sequence generation strategy. It is supported in Oracle 10g drivers only.

Id Primary Key Generator Nhibernate Download

Next TopicDialects In Hibernate

Unique Key


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