-- 006_accommodation_history_constraint.sql
-- 目标：移除 train_accommodation 全局唯一约束 uq_accommodation_enroll，
--       创建部分唯一索引 uq_accommodation_enroll_active (每个 enrollment 最多 1 条有效住宿)，
--       允许学员拥有多条退宿历史记录 (status=2)，彻底支持调房与历史留痕。

DO $$
DECLARE
    v_dup_count INTEGER;
    v_con_def TEXT;
    v_idx_def TEXT;
BEGIN
    -- 1. Pre-Migration Data Safety Guard
    -- 检查是否已存在同一个 enrollment_id 拥有 2 条或以上处于有效状态 (0=预分配, 1=已入住) 的住宿记录
    SELECT COUNT(*) INTO v_dup_count
    FROM (
        SELECT enrollment_id
        FROM train_accommodation
        WHERE accommodation_status IN (0, 1)
        GROUP BY enrollment_id
        HAVING COUNT(*) > 1
    ) t;

    IF v_dup_count > 0 THEN
        RAISE EXCEPTION 'Pre-migration check failed: found % enrollment(s) with multiple active accommodations (status IN (0,1)). Manual resolution required before applying migration 006.', v_dup_count;
    END IF;

    -- 2. Existing Constraint Guard (Idempotent Drop)
    -- 检查约束 uq_accommodation_enroll 是否存在且符合预期
    SELECT pg_get_constraintdef(c.oid) INTO v_con_def
    FROM pg_constraint c
    JOIN pg_class t ON c.conrelid = t.oid
    WHERE t.relname = 'train_accommodation'
      AND c.conname = 'uq_accommodation_enroll';

    IF v_con_def IS NOT NULL THEN
        -- 验证旧约束是否为 UNIQUE (enrollment_id)
        IF v_con_def ILIKE 'UNIQUE (enrollment_id)%' THEN
            RAISE NOTICE 'Dropping legacy global unique constraint: uq_accommodation_enroll';
            ALTER TABLE train_accommodation DROP CONSTRAINT uq_accommodation_enroll;
        ELSE
            RAISE EXCEPTION 'Constraint collision: constraint uq_accommodation_enroll exists but definition is unexpected: %', v_con_def;
        END IF;
    ELSE
        RAISE NOTICE 'Constraint uq_accommodation_enroll does not exist, skipping drop.';
    END IF;

    -- 3. New Index Collision Guard & Creation
    -- 检查部分唯一索引 uq_accommodation_enroll_active
    SELECT pg_get_indexdef(i.indexrelid) INTO v_idx_def
    FROM pg_index i
    JOIN pg_class c ON i.indexrelid = c.oid
    JOIN pg_class t ON i.indrelid = t.oid
    WHERE t.relname = 'train_accommodation'
      AND c.relname = 'uq_accommodation_enroll_active';

    IF v_idx_def IS NOT NULL THEN
        -- 验证已有索引是否为预期的部分唯一索引
        IF v_idx_def ILIKE '%UNIQUE INDEX uq_accommodation_enroll_active ON %train_accommodation%enrollment_id%WHERE%accommodation_status%' THEN
            RAISE NOTICE 'Partial unique index uq_accommodation_enroll_active already exists with expected definition, skipping creation.';
        ELSE
            RAISE EXCEPTION 'Index collision: index uq_accommodation_enroll_active exists but definition differs: %', v_idx_def;
        END IF;
    ELSE
        RAISE NOTICE 'Creating partial unique index: uq_accommodation_enroll_active';
        CREATE UNIQUE INDEX uq_accommodation_enroll_active
        ON train_accommodation (enrollment_id)
        WHERE accommodation_status IN (0, 1);
    END IF;
END $$;