In order to retrieve a specific repository item, you need its repository ID. The primary table defines an id property, which specifies the repository ID of repository items of this item type. You can specify this property using the id-column-names attribute, and you do not need to further define the id property in the item descriptor. If you do not define an id property in the item descriptor, then the id property must use the default data-type, which is string. However, if you do explicitly define the id property in the item descriptor, using a <property> tag, you can query repository items by their ID and you can set a different data-type for the id property. The columns specified by the id-column-names attribute do not have to use the same data-type as each other; you can have a composite repository ID the elements of which are strings, integers, and longs.
After a repository item is created, but before it is added to the database, you can change its repository ID by changing the value of the id property. Once the item has become persistent, you can no longer change the ID.
