I didn't like the AUTO generation strategy (which resulted in a SEQUENCE) used by the sample implementation, so here is an EclipseLink orm.xml you can use to change them to an IDENTITY generation strategy.
<?xml version="1.0" encoding="UTF-8"?>
<entity-mappings version="2.1"
xmlns="http://www.eclipse.org/eclipselink/xsds/persistence/orm"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
<entity class="org.picketlink.idm.jpa.model.sample.simple.DigestCredentialTypeEntity">
<attributes>
<id name="id">
<generated-value strategy="IDENTITY" />
</id>
</attributes>
</entity>
<entity class="org.picketlink.idm.jpa.model.sample.simple.OTPCredentialTypeEntity">
<attributes>
<id name="id">
<generated-value strategy="IDENTITY" />
</id>
</attributes>
</entity>
<entity class="org.picketlink.idm.jpa.model.sample.simple.PasswordCredentialTypeEntity">
<attributes>
<id name="id">
<generated-value strategy="IDENTITY" />
</id>
</attributes>
</entity>
<entity class="org.picketlink.idm.jpa.model.sample.simple.X509CredentialTypeEntity">
<attributes>
<id name="id">
<generated-value strategy="IDENTITY" />
</id>
</attributes>
</entity>
<entity class="org.picketlink.idm.jpa.model.sample.simple.AttributeTypeEntity">
<attributes>
<id name="id">
<generated-value strategy="IDENTITY" />
</id>
</attributes>
</entity>
<entity class="org.picketlink.idm.jpa.model.sample.simple.RelationshipIdentityTypeEntity">
<attributes>
<id name="identifier">
<generated-value strategy="IDENTITY" />
</id>
</attributes>
</entity>
</entity-mappings>