
4.1 Object Types
As discussed in the previous chapter, an ORM object type is either a value type or an
entity type. A value corresponds to an entry in a table, and is typically a character string
or numeric constant (e.g., “Ireland,” 56). A value identifies or refers to itself, so there
is no need to specify a reference scheme (identification scheme) for it.
An entity is identified by means of a reference scheme that uses one or more values.
A simple reference scheme uses only one value (e.g., the Country with CountryCode
‘US’). A good rule of thumb is “If you can write it down, it’s a value.” Clearly, you can
write down the code ‘US,’ but you can’t ...