In any case I generally avoid using derived attributes altogether and
use custom business logic instead for the following reasons
documented in the EOModeler user's guild:
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Derived attributes are effectively read-only since there is no place
to write them back to. You could get the value of a derived attribute
and write it back to another column but that requires another
attribute. And if you need to store the value of a derived attribute,
it’s usually much better to perform the derivation in business logic
rather than at the attribute level. (Alternatively, you could use
custom read and write formats to accomplish this. See “Read Format
and Write Format”). By deriving attributes at the business-logic
level, you write the code in Java, you avoid writing database-
specific SQL, and you get the full benefits of enterprise objects.
One of the most important benefits is the internal update
notifications that enterprise objects send and receive. In the
previous example, if you change an employee’s monthly salary, the
derived attribute that calculates the annual salary is then
incorrect. And since the attribute is derived, its value as it exists
in the enterprise object is immutable. Unless the object is flushed
from the access layer’s snapshot and refreshed, the derived attribute
is stale and inaccurate.
Derived attributes can be useful but should probably be reserved for
read-only applications and can usually be replaced by values derived
in business logic. Also, because derived attributes don’t directly
map to anything in the database, they cannot be used for locking or
as primary keys.
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Robert Walker
[EMAIL PROTECTED]
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