-- ═══════════════════════════════════════════════════════════════════════════
-- JUNCTION REVAMP — Units and Lessons run independently        (Jul. 7, 2026)
--
--   courses ⇄ course_units ⇄ units ⇄ unit_lessons ⇄ lessons
--
-- Run order matters: create → backfill → relax progress FKs → drop old columns.
-- Postgres / CockroachDB compatible. Take a backup before running.
-- ═══════════════════════════════════════════════════════════════════════════

BEGIN;

-- ── 1. Junction tables ───────────────────────────────────────────────────────

CREATE TABLE course_units (
  course_unit_id BIGINT       GENERATED BY DEFAULT AS IDENTITY PRIMARY KEY,
  course_id      BIGINT       NOT NULL REFERENCES courses (course_id) ON DELETE CASCADE,
  unit_id        BIGINT       NOT NULL REFERENCES units   (unit_id)   ON DELETE CASCADE,
  order_index    INTEGER      NOT NULL DEFAULT 0,
  "createdBy"    BIGINT       NULL,
  "updatedBy"    BIGINT       NULL,
  "createdAt"    TIMESTAMPTZ  NOT NULL DEFAULT CURRENT_TIMESTAMP,
  "updatedAt"    TIMESTAMPTZ  NOT NULL DEFAULT CURRENT_TIMESTAMP,
  CONSTRAINT uq_course_units_course_unit UNIQUE (course_id, unit_id)
);

CREATE INDEX idx_course_units_course_id ON course_units (course_id);
CREATE INDEX idx_course_units_unit_id   ON course_units (unit_id);
CREATE INDEX idx_course_units_order     ON course_units (order_index);

CREATE TABLE unit_lessons (
  unit_lesson_id BIGINT       GENERATED BY DEFAULT AS IDENTITY PRIMARY KEY,
  unit_id        BIGINT       NOT NULL REFERENCES units   (unit_id)   ON DELETE CASCADE,
  lesson_id      BIGINT       NOT NULL REFERENCES lessons (lesson_id) ON DELETE CASCADE,
  order_index    INTEGER      NOT NULL DEFAULT 0,
  "createdBy"    BIGINT       NULL,
  "updatedBy"    BIGINT       NULL,
  "createdAt"    TIMESTAMPTZ  NOT NULL DEFAULT CURRENT_TIMESTAMP,
  "updatedAt"    TIMESTAMPTZ  NOT NULL DEFAULT CURRENT_TIMESTAMP,
  CONSTRAINT uq_unit_lessons_unit_lesson UNIQUE (unit_id, lesson_id)
);

CREATE INDEX idx_unit_lessons_unit_id   ON unit_lessons (unit_id);
CREATE INDEX idx_unit_lessons_lesson_id ON unit_lessons (lesson_id);
CREATE INDEX idx_unit_lessons_order     ON unit_lessons (order_index);

-- ── 2. Backfill from the old direct FKs (preserves ordering) ─────────────────

INSERT INTO course_units (course_id, unit_id, order_index, "createdBy", "createdAt", "updatedAt")
SELECT u.course_id, u.unit_id, COALESCE(u.order_index, 0), u."createdBy", CURRENT_TIMESTAMP, CURRENT_TIMESTAMP
FROM units u
WHERE u.course_id IS NOT NULL;

INSERT INTO unit_lessons (unit_id, lesson_id, order_index, "createdBy", "createdAt", "updatedAt")
SELECT l.unit_id, l.lesson_id, COALESCE(l.order_index, 0), l."createdBy", CURRENT_TIMESTAMP, CURRENT_TIMESTAMP
FROM lessons l
WHERE l.unit_id IS NOT NULL;

-- ── 3. Progress tables — standalone reads carry NULL course/unit context ─────

ALTER TABLE unit_reading_progress   ALTER COLUMN course_id DROP NOT NULL;
ALTER TABLE lesson_reading_progress ALTER COLUMN course_id DROP NOT NULL;
ALTER TABLE lesson_reading_progress ALTER COLUMN unit_id   DROP NOT NULL;

-- ── 4. Drop the old FK + order columns ───────────────────────────────────────
-- (dependent indexes/constraints on these columns are dropped automatically)

ALTER TABLE units   DROP COLUMN course_id;
ALTER TABLE units   DROP COLUMN order_index;
ALTER TABLE lessons DROP COLUMN unit_id;
ALTER TABLE lessons DROP COLUMN order_index;

COMMIT;

-- ── Sanity checks (run after commit) ─────────────────────────────────────────
-- SELECT COUNT(*) FROM course_units;   -- should equal old COUNT(*) FROM units WHERE course_id IS NOT NULL
-- SELECT COUNT(*) FROM unit_lessons;   -- should equal old COUNT(*) FROM lessons WHERE unit_id IS NOT NULL
-- SELECT c.title, u.title, cu.order_index FROM course_units cu
--   JOIN courses c USING (course_id) JOIN units u USING (unit_id)
--   ORDER BY c.title, cu.order_index LIMIT 20;