ADR-007: Subcontracting Strategy — LegAssignment Nullability

Status: ✅ Accepted Date: 2026-04-09 Triggered by: Level 2 Audit — Crew & Fleet Management (Finding S1)

Context

Bus tour operators in the DACH region regularly outsource individual service legs to partner bus companies (Fremdanmietung) during capacity bottlenecks or for specialized routes. The leg_assignments table in the Operations schema links a ServiceLeg to an internal Vehicle and CrewMember. Both fields are currently Not Null, which prevents modeling subcontracted legs.

PRODUCT_schema-feature-gaps.md §4 proposes adding supplier_id and making the internal asset fields nullable. Two options were evaluated:

• Option A — Nullable FKs + supplier_id: Minimal schema change. A CHECK constraint enforces mutual exclusivity: (supplier_id IS NOT NULL) OR (vehicle_id IS NOT NULL AND crew_member_id IS NOT NULL).
• Option B — Separate SubcontractedLeg entity: Cleaner domain separation but duplicates assignment structure and complicates dispatch board queries.

Decision

Option A — nullable FKs with CHECK constraint on leg_assignments.

Rationale: Subcontracting is a Phase 2 feature with medium priority. Option A keeps the schema simple and avoids a parallel entity that the dispatch board would need to union-query. If subcontracting becomes a primary workflow (e.g., marketplace model), migration to Option B can be done without breaking the existing assignment structure.

Consequences

• leg_assignments.vehicle_id and crew_member_id become nullable.
• leg_assignments.supplier_id added as soft FK to backoffice.suppliers.
• CHECK constraint: (supplier_id IS NOT NULL) OR (vehicle_id IS NOT NULL AND crew_member_id IS NOT NULL).
• Dispatch board renders subcontracted legs as "Outsourced" rows with distinct styling.
• Operations API must validate the mutual exclusivity on write (in addition to the DB constraint).
• No cross-context event needed — supplier_id is a soft FK read at query time.