Note that each Film object refers to a FilmCategory object by that object's unique object identifier, or OID. For instance, the “Drama” object has an OID of 104, which two of the Film objects use to refer to the appropriate FilmCategory object.
Typically, the OID is not “real” data. That is, the OID is not a property that the application developer defines or sets directly. Instead, the system automatically assigns a unique value whenever a new object is created. This is easy for the developer, extremely efficient, and has the added benefit of enabling the values of all “real” properties to be changed.
We could, if necessary, use a real property as the OID, but that property would, by necessity, be unchangeable.