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From Apache Wiki <wikidi...@apache.org>
Subject [Ws Wiki] Update of "ForeignKeyRepresentationAndManagement" by KevinWilliams
Date Fri, 05 Jan 2007 21:45:23 GMT
Dear Wiki user,

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The following page has been changed by KevinWilliams:
http://wiki.apache.org/ws/ForeignKeyRepresentationAndManagement

New page:
Database table foreign keys map to !DataObjects as relationships to other !DataObjects.  Consider
two tables: DEPARTMENT and EMPLOYEE and assume the EMPLOYEE table has a FK column DEPT_ID
that references a row in the DEPARTEMENT table.

If you us the DAS to read a particular department and all of its related employees then you
will end up with a DEPARTMENT !DataObject that has a property named "EMPLOYEE" that is a reference
to a list of related EMPLOYEE !DataObjects.  You can navigate dorectly to the employees via
this property like this:  department.getList("EMPLOYEE").

Although the relationship is mapped to the !DataObjects as a relationship, the FK value is
also stored in the repsective !DataObject.  So, continuing with this example, the EMPLOYEE
!DataObject will have a simple property named "DEPT_ID" that holds the value of the FK.  This
property is not maintained after the read.  So, if an EMPLOYEE !DataObject is moved to a different
department, the FK property in EMPLOYEE will not be modified.  Although most applications
will ignore this FK property, there are a few scenarios that make use of it.  In fact, although
the preferred method of modifying relationships between !DataObjects in a data graph is to
use normal POJO manipulation, the DAS does allow the modification of a relationship via direct
modification of the FK value.

When the DAS is scanning changes made to a data graph, it employs the following set of rule
to guide the generation of update statements:

"The value for a foreign key column will come from either the primary key of the parent dataobject
or from the FK property itself. If the relationship is set but the FK property is not, the
value will be taken from the parent's primary key. If the FK property is set but the relationship
is not, the value will be taken from the property value. If both the relationship and the
FK property are set, an exception will be thrown stating that "Foreign key properties should
not be set when the corresponding relationship has changed"

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