Classes that use application identity require slightly different extensions than those enumerated above. First, no pk-column extension is ever needed, since the primary key columns can be determined from the columns for the primary key fields. Second, because application identity classes may use multiple primary key columns, all extensions associated with primary keys must be prefixed with the name of the corresponding field. This applies to all ref-column extensions, and to data-column extensions for one-to-one and many-to-many relation fields. A concrete example using application identity is presented in the next section.