# Automatic Primary Key Generation

In WebLogic it is possible to automatically generate a primary key to be used when creating a new CMP entity bean instead of providing primary key values. You can auto-generate primary keys in various vendor-specific ways - using Oracle, SQLServer, or SQLServer2000 - or you can use a vendor-neutral named sequence table. In all cases auto-generated primary keys are of type Integer or Long.

The topics in this section are:

- Primary Key Generation Using Oracle's Sequence
- Primary Key Generation Using SQL Server's IDENTITY
- Primary Key Generation Using a Named Sequence Table
- Defining the CMP Entity Bean

##
Primary Key Generation Using Oracle's Sequence

Oracle provides the 'sequence' utility to automatically generate unique primary keys. To use this utility to auto-generate primary keys for a CMP entity bean, you must create a sequence table and use the ejbgen:automatic-key-generation tag to point to this table.

In your Oracle database, you must create a sequence table that will create the primary keys, like is shown in the following example:

    create sequence myOracleSequence
    start with 1
    nomaxvalue;

This creates a sequences of primary key, starting with 1, followed by 2, 3, and so forth. The sequence table in the example uses the default increment 1, but you can change this by specifying the increment keyword, such as increment by 3. When you do the latter, you must specify the exact same value in the cache-size attribute of the ejbgen:automatic-key-generation tag:

    @ejbgen:automatic-key-generation type="Oracle" name="myOracleSequence" cache-size="3"

If you have specified automatic table creation in the CMP bean's project settings, the sequence table will be created automatically when the entity bean is deployed. For more information, see @ejbgen:jar-settings Annotation. For more information on the definition of a CMP entity bean, see below.

## Primary Key Generation Using SQL Server's IDENTITY

In SQL Server (2000) you can use the 'IDENTITY' keyword to indicate that a primary-key needs to be auto-generated. The following example shows a common scenario where the first primary key value is 1, and the increment is 1:

    CREATE TABLE Customer (Customer_ID int IDENTITY(1,1), FirstName varchar(30) LastName varchar(30))

In the CMP entity bean definition you need to specify SQLServer(2000) as the type of automatic key generator you are using. You can also provide a cache size:

    @ejbgen:automatic-key-generation type="SQLServer"

If you have specified automatic table creation in the CMP bean's project settings, the sequence table will be created automatically when the entity bean is deployed. For more information, see @ejbgen:jar-settings Annotation. For more information on the definition of a CMP entity bean, see below.

Note. The SQLServer2000 option is the same as SQLServer, except that SQLServer uses @@IDENTITY column to get the generated key value and SQLServer2000 uses the SCOPE_IDENTITY() function instead.

## Primary Key Generation Using a Named Sequence Table

A named sequence table is similar to the Oracle sequence functionality in that a dedicated table is used to generate primary keys. However, the named sequence table approach is vendor-neutral. To auto-generate primary keys this way, create a named sequence table using the two SQL statements shown in the example:

    CREATE Table MyNamedSequence (SEQUENCE number);

    INSERT into MyNamedSequence VALUES (0);

In the CMP entity bean definition you need to specify the named sequence table as the type of automatic key generator you are using. You can also provide a cache size:

    @ejbgen:automatic-key-generation name="MySequence" type="MyNamedSequenceTable" cache-size="100"

If you have specified automatic table creation in the CMP bean's project settings, the sequence table will be created automatically when the entity bean is deployed. For more information, see @ejbgen:jar-settings Annotation. For more information on the definition of a CMP entity bean, see the next section.

Note. When you specify a cache-size for a named sequence table, a series of unique values are reserved for entity bean creation. When a new cache is necessary, a second series of unique values is reserved, under the assumption that the first series of unique values was entirely used. This guarantees that primary key values are always unique, although it leaves open the possibility that primary key values are not necessarily sequential. For instance, when the first series of values is 10...20, the second series of values is 21-30, even if not all values in the first series were actually used to create entity beans.

## Defining the CMP Entity Bean

When defining a CMP entity bean that uses one of the primary key generators, you point to the name of the primary key generator table to obtain primary keys, using the ejbgen:automatic-key-generation tag. Also, you must define a primary key field of type Integer or Long, to set and get the auto-generated primary key. However, the ejbCreate method does not take a primary key value as an argument. Instead the EJB container adds the correct primary key to the entity bean record.

The following example shows what the entity bean might look like. Notice that the bean uses the named sequence option describe above, and that ejbCreate method does not take a primary key:

     /**
     * @ejbgen:automatic-key-generation name="MyNamedSequence" type="NamedSequenceTable" cache-size="100"
     *
     * @ejbgen:entity prim-key-class="java.lang.Integer"
     *   ejb-name = "Customer"
     *   data-source-name = "MyCustomerDataSource"
     *   table-name = "customer"
     *   abstract-schema-name = "Customer"
     *
     * ...
     */
    abstract public class Customer extends GenericEntityBean implements EntityBean
    {
        ...

        /**
         * @ejbgen:cmp-field primkey-field="true" column="Customer_ID"
         * @ejbgen:local-method
         */
        public abstract Integer getCustomer_ID();

        /**
         * @ejbgen:local-method
         */
        public abstract void setCustomer_ID(Integer arg);

        public java.lang.Integer ejbCreate(java.lang.String LastName, java.lang.String FirstName)
        {
          setLastName(LastName);
          setFirstName(FirstName);

          return null; // FIXME return PK value
        }
    }

Related Topics

@ejbgen:automatic-key-generation Annotation

Automatic Primary Key Generation Sample

@ejbgen:jar-settings Annotation