Foreign keys enforce referential integrity between related tables

Without foreign key constraints, the database allows orphan rows — comments that reference deleted posts, orders that reference nonexistent users, line items whose product was removed. Application code must then paper over these inconsistencies with defensive null checks, producing unpredictable behavior in edge cases. Missing ON DELETE clauses mean the database default (NO ACTION) silently blocks parent deletes or, in some configurations, leaves orphans. ISO 25010 functional-correctness requires that referential constraints are enforced at the storage layer, not assumed from application logic that can be bypassed by direct SQL access, background jobs, or migration scripts.

Add @relation with explicit onDelete in Prisma, or FOREIGN KEY ... REFERENCES ... ON DELETE in raw SQL, for every column that logically points to another table.

model Comment {
  id     String @id @default(uuid())
  postId String
  post   Post   @relation(fields: [postId], references: [id], onDelete: Cascade)
}

ALTER TABLE orders
  ADD CONSTRAINT fk_orders_user
  FOREIGN KEY (user_id)
  REFERENCES users(id)
  ON DELETE RESTRICT;

Choose CASCADE for owned data (comments belong to a post), SET NULL for optional references, and RESTRICT for financial or audit records where parent deletion must be blocked.