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 write down the actual country that is referred to by the code US. Thus, the country code US is a value, but the actual country (the United States of America) is an entity.

Entities and values in an ORM model correspond to real world objects in the application domain, not tables and columns in a logical database model. Trying to make this association will only lead to confusion.

A simple reference scheme for an entity type may be declared explicitly using a binary association (e.g., Country has CountryCode. This reference scheme may be abbreviated by using a reference mode displayed in parenthesis after the entity type name (e.g., Country(Code).

A composite reference scheme uses two or more binary associations, e.g., the State...