PostgreSQL Polymorphic Associations: Why One Column Pair Creates Data Integrity Risks

Polymorphic associations, common in frameworks like Rails, Laravel, and Django, store a parent type name and ID in two columns on a child table without any real foreign key constraint. PostgreSQL cannot enforce referential integrity across multiple parent tables from a single column, meaning orphaned records, invalid IDs, and mistyped class names can accumulate silently. When the set of parent tables is small and fixed, the recommended approach is one nullable foreign key per parent combined with a CHECK constraint ensuring exactly one is populated. For larger or more varied parent sets, a shared supertype table restores proper database-level enforcement. The polymorphic column pair should only be retained when the list of parent entities is genuinely open-ended, and its virtual relationships should be explicitly documented in schema diagrams.