-- curriculum_assignments — which curriculum path a cohort or an individual student is on.
-- Only one curriculum exists today (scenarios/rootnode-path.json, "RootNode Learning Path") —
-- this table exists so assigning it is a real, visible instructor action (and so a second path
-- can be added later without a schema change), not because multiple paths exist yet.

CREATE TABLE IF NOT EXISTS curriculum_assignments (
  assignment_id    BIGSERIAL PRIMARY KEY,
  cohort_id        TEXT REFERENCES cohorts(cohort_id),
  student_id       TEXT REFERENCES students(student_id),
  curriculum_path  TEXT NOT NULL DEFAULT 'rootnode-path',
  assigned_at      TIMESTAMPTZ NOT NULL DEFAULT now(),
  CONSTRAINT curriculum_assignment_target CHECK (cohort_id IS NOT NULL OR student_id IS NOT NULL)
);

CREATE INDEX IF NOT EXISTS idx_curriculum_cohort ON curriculum_assignments(cohort_id);
CREATE INDEX IF NOT EXISTS idx_curriculum_student ON curriculum_assignments(student_id);