-- Migration 034: Progressionskanten optional mit Übungsvarianten (Knoten = Übung oder konkrete Variante).
-- UNIQUE und CHECK angepasst; Kanten innerhalb derselben Übung nur zwischen verschiedenen Varianten.

ALTER TABLE exercise_progression_edges
    ADD COLUMN IF NOT EXISTS from_exercise_variant_id INT REFERENCES exercise_variants(id) ON DELETE CASCADE,
    ADD COLUMN IF NOT EXISTS to_exercise_variant_id INT REFERENCES exercise_variants(id) ON DELETE CASCADE;

CREATE INDEX IF NOT EXISTS idx_progression_edges_from_variant ON exercise_progression_edges(from_exercise_variant_id)
    WHERE from_exercise_variant_id IS NOT NULL;
CREATE INDEX IF NOT EXISTS idx_progression_edges_to_variant ON exercise_progression_edges(to_exercise_variant_id)
    WHERE to_exercise_variant_id IS NOT NULL;

DO $$
BEGIN
    ALTER TABLE exercise_progression_edges
        DROP CONSTRAINT exercise_progression_edges_graph_id_from_exercise_id_to_exercise_id_edge_type_key;
EXCEPTION
    WHEN undefined_object THEN NULL;
END $$;

DO $$
BEGIN
    ALTER TABLE exercise_progression_edges DROP CONSTRAINT exercise_progression_edges_check;
EXCEPTION
    WHEN undefined_object THEN NULL;
END $$;

ALTER TABLE exercise_progression_edges ADD CONSTRAINT exercise_progression_edges_endpoints_distinct CHECK (
    (from_exercise_id <> to_exercise_id)
    OR (
        from_exercise_variant_id IS NOT NULL
        AND to_exercise_variant_id IS NOT NULL
        AND from_exercise_variant_id <> to_exercise_variant_id
    )
);

CREATE UNIQUE INDEX IF NOT EXISTS exercise_progression_edges_unique_endpoints
ON exercise_progression_edges (
    graph_id,
    from_exercise_id,
    COALESCE(from_exercise_variant_id, 0),
    to_exercise_id,
    COALESCE(to_exercise_variant_id, 0),
    edge_type
);