Default Database Transformations
In this section we describe what we called default database
conversion functions. If no user-defined conversion
functions are specified (see Section 2.2), the system transforms
the objects in the database using default transformation
rules. When a class in the schema is modified, the
system compares each attribute of the class before and after
the modification of the class and transforms the values
of the object attributes according to the default rules as
followsi :
An attribute defined in a class before its modification
and non present in the class after the modification
(i.e. a deleted attribute) is ignored.
An attribute which is not present in a class before its
modification and present after its modification (i.e. a
new attribute) is initialized with default initial values
(i.e. 0 for an integer attribute, nil for an attribute
referring to a class, etc.).
An attribute present in both the class before the
change and after the change is transformed according
to the rules in Table 1.