Summary
If you get
"Parent Keys Not Found" error when you try to enable/create relation from a child table to parent table, then you must identify and remove bad 'child' records.
The records can be identified and
removed using the query:
DELETE FROM childtablename ct
WHERE NOT EXISTS (SELECT 'x' FROM parenttablename pt
WHERE ct.keycolumn1 = pt.keycolumn1 AND ct.keycolumn2 = pt.keycolumn2...);
