JPA : Datastore Identifiers

A datastore identifier is a simple name of a database object, such as a column, table, index, or view, and is composed of a sequence of letters, digits, and underscores ( _ ) that represents it's name. DataNucleus allows users to specify the names of tables, columns, indexes etc but if the user doesn't specify these DataNucleus will generate names. Generation of identifier names is controlled by an IdentifierFactory, and DataNucleus provides a default implementation. You can provide your own IdentifierFactory plugin to give your own preferred naming if so desired. You set the IdentifierFactory by setting the persistence property datanucleus.identifierFactory . Set it to the symbolic name of the factory you want to use. JPA defines what datastore identifiers should default to when not specified. DataNucleus provides a factory that meets this requirement.

  • jpa IdentifierFactory (default for JPA persistence)

In describing the different possible naming conventions available out of the box with DataNucleus we'll use the following example 

class MyClass
{
    String myField1;
    Collection<MyElement> elements1; // Using join table
    Collection<MyElement> elements2; // Using foreign-key
}

class MyElement
{
    String myElementField;
    MyClass myClass2;
}


IdentifierFactory 'jpa'

The IdentifierFactory "jpa" aims at providing a naming policy consistent with the JPA specification.

Using the same example above, the rules in this IdentifierFactory mean that, assuming that the user doesnt specify any <column> elements :-

  • MyClass will be persisted into a table named MYCLASS
  • When using datastore identity MYCLASS will have a column called MYCLASS_ID
  • MyClass.myField1 will be persisted into a column called MYFIELD1
  • MyElement will be persisted into a table named MYELEMENT
  • MyClass.elements1 will be persisted into a join table called MYCLASS_MYELEMENT
  • MYCLASS_ELEMENTS1 will have columns called MYCLASS_MYCLASS_ID (FK to owner table) and ELEMENTS1_ELEMENT_ID (FK to element table)
  • MyClass.elements2 will be persisted into a column ELEMENTS2_MYCLASS_ID (FK to owner) table
  • Any discriminator column will be called DTYPE
  • Any index column in a List for field MyClass.myField1 will be called MYFIELD1_ORDER
  • Any adapter column added to a join table to form part of the primary key will be called IDX
  • Any version column for a table will be called VERSION
IdentifierFactory - Controlling the Case

The underlying datastore will define what case of identifiers are accepted. By default, DataNucleus will capitalise names (assuming that the datastore supports it). You can however influence the case used for identifiers. This is specifiable with the persistence property datanucleus.identifier.case , having the following values

  • UpperCase: identifiers are in upper case
  • LowerCase: identifiers are in lower case
  • PreserveCase: No case changes are made to the name of the identifier provided by the user (class name or jdo metadata).

Please be aware that some datastores only support UPPERCASE or lowercase identifiers and so setting this parameter may have no effect if your database doesn't support that option.