-- =============================================
-- Education 模块 — 收藏夹 DDL
-- Ticket #10: 学生收藏题目
-- Migration: 007
-- Prerequisites: 000-education-schema.sql (base tables)
-- =============================================

-- =============================================
-- Preconditions
-- =============================================
-- Operator is expected to verify:
--   SELECT COUNT(*) FROM information_schema.tables
--     WHERE table_schema = DATABASE()
--       AND table_name = 'education_favorite';
-- Result MUST be 0 before executing this migration.

-- =============================================
-- 收藏表
-- =============================================
-- Purpose: Student favorites for questions with safe snapshots.
--   Each (tenant, user, target_type, target_id) is a unique entry.
--   Logical deletion: setting deleted=1 marks as unfavorited.
--   Re-adding after deletion reactivates the row via ON DUPLICATE KEY UPDATE.
--
-- target_type values: 'QUESTION' (extensible enum)
--
-- Snapshot fields (stem, type, difficulty, options, content_version):
--   populated at creation time from the visible question's safe fields.
--   These snapshots preserve the question state as it appeared when favorited,
--   and remain stable even if the source question later changes or becomes unavailable.
--
-- available flag:
--   FALSE when the source question becomes hidden/unpublished after being
--   favorited. Existing favorites with available=FALSE remain listable but
--   display an "unavailable" indicator. New favorites cannot be created for
--   unavailable resources.
--
-- Indexes:
--   uk_tenant_user_target — per (tenant, user, target_type, target_id) uniqueness.
--     INSERT ... ON DUPLICATE KEY UPDATE is the primary reactivation path.
--   idx_tenant_user — covers listing queries filtered by current tenant+user.
--   idx_tenant_user_target_type — covers target-type-filtered listing.
CREATE TABLE `education_favorite` (
    `id`               BIGINT       NOT NULL AUTO_INCREMENT COMMENT '主键',
    `tenant_id`        BIGINT       NOT NULL              COMMENT '租户编号',
    `user_id`          BIGINT       NOT NULL              COMMENT '学生用户编号',
    `target_type`      VARCHAR(32)  NOT NULL              COMMENT '目标类型：QUESTION',
    `target_id`        VARCHAR(64)  NOT NULL              COMMENT '目标 ID（题目 ID）',
    -- safe snapshot fields
    `stem`             TEXT         DEFAULT NULL          COMMENT '题干快照',
    `type`             VARCHAR(32)  DEFAULT NULL          COMMENT '题型快照',
    `difficulty`       VARCHAR(32)  DEFAULT NULL          COMMENT '难度快照',
    `options`          JSON         DEFAULT NULL          COMMENT '选项快照 JSON（不含 isCorrect）',
    `content_version`  VARCHAR(64)  NOT NULL DEFAULT ''   COMMENT '题目内容版本',
    -- availability
    `available`        BIT(1)       NOT NULL DEFAULT b'1' COMMENT '源资源是否可用',
    -- audit
    `creator`          VARCHAR(64)  DEFAULT ''            COMMENT '创建者',
    `create_time`      DATETIME     NOT NULL DEFAULT CURRENT_TIMESTAMP COMMENT '创建时间',
    `updater`          VARCHAR(64)  DEFAULT ''            COMMENT '更新者',
    `update_time`      DATETIME     NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP COMMENT '更新时间',
    `deleted`          BIT(1)       NOT NULL DEFAULT b'0' COMMENT '是否删除',
    PRIMARY KEY (`id`),
    UNIQUE KEY `uk_tenant_user_target` (`tenant_id`, `user_id`, `target_type`, `target_id`),
    KEY `idx_tenant_user` (`tenant_id`, `user_id`),
    KEY `idx_tenant_user_target_type` (`tenant_id`, `user_id`, `target_type`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci COMMENT='教育-收藏夹';

-- =============================================
-- Post-migration verification queries
-- =============================================
-- Verify new table exists:
--   SHOW CREATE TABLE education_favorite;
-- Verify unique key is enforced:
--   SHOW INDEX FROM education_favorite WHERE Key_name = 'uk_tenant_user_target';
-- Verify no orphan data (should be 0 after fresh migration):
--   SELECT COUNT(*) FROM education_favorite;