When persisting objects, the objects require an "identity", so as to uniquely identify them for storage and retrieval. This in-memory id is normally mapped directly to a primary key column or columns in the database.
Every object in an OpenAccess ORM database has a unique identifier, i.e. a "primary key" from the database point of view. The identifier has to be defined when the object is made persistent (except when a key generator mechanism is used). Using this identifier, you can directly retrieve an object from the database.
Types of Object Identity