competition-website/database/migrations/021-unique-submission-indexes.js

'use strict';

/**
 * 迁移 021：为答题提交表增加唯一约束，杜绝并发作答重复落库（Bug #12 / B12）
 *
 * 背景（B12）：原 saveAnswers 使用「先 findOne 再 create」的非原子写法，且
 * (aggregation_uid, question_uid, question_type) 仅为普通索引。同一用户并发提交
 * 时，多个请求都 find 不到记录，于是各自 create，导致同一题被插入多行
 * （实测：3 路并发产生 57 行 / 20 题，exam_submissions 出现 3 条同选手记录）。
 * 重复行会污染 answeredCount、判分明细与成绩恢复，属数据一致性缺陷。
 *
 * 修复：
 * 1) zhongzhi_submissions / gaozhi_submissions 的复合索引升级为唯一索引
 *    uk_{table}_submission (aggregation_uid, question_uid, question_type)；
 *    service.saveAnswers 改用 upsert(conflictFields=[...])，由 MySQL 的
 *    INSERT ... ON DUPLICATE KEY UPDATE 在并发下原子折叠为单行更新。
 * 2) exam_submissions 的 contestant_id 索引升级为唯一索引
 *    uk_exam_submission_contestant，配合 service 中并发安全的
 *    _ensureExamSubmissionSafe，保证每选手仅一条提交记录。
 *
 * 注意：up/down 均先去重（保留最小 id）再加减索引，确保幂等且不会因历史脏数据失败。
 */
module.exports = {
  up: async (queryInterface) => {
    // ---- 去重：同 (aggregation_uid, question_uid, question_type) 仅留最小 id ----
    await queryInterface.sequelize.query(`
      DELETE t FROM zhongzhi_submissions t
      WHERE t.id NOT IN (
        SELECT min_id FROM (
          SELECT MIN(id) AS min_id FROM zhongzhi_submissions
          GROUP BY aggregation_uid, question_uid, question_type
        ) AS keep
      )
    `);
    await queryInterface.sequelize.query(`
      DELETE t FROM gaozhi_submissions t
      WHERE t.id NOT IN (
        SELECT min_id FROM (
          SELECT MIN(id) AS min_id FROM gaozhi_submissions
          GROUP BY aggregation_uid, question_uid, question_type
        ) AS keep
      )
    `);
    // exam_submissions：每 contestant_id 仅留最小 id
    await queryInterface.sequelize.query(`
      DELETE t FROM exam_submissions t
      WHERE t.id NOT IN (
        SELECT min_id FROM (
          SELECT MIN(id) AS min_id FROM exam_submissions
          GROUP BY contestant_id
        ) AS keep
      )
    `);

    // ---- zhongzhi ----
    await queryInterface.removeIndex('zhongzhi_submissions', 'idx_zhongzhi_submissions_agg_qtype').catch(() => {});
    await queryInterface.addIndex('zhongzhi_submissions', {
      name: 'uk_zhongzhi_submission',
      unique: true,
      fields: ['aggregation_uid', 'question_uid', 'question_type'],
    }).catch(() => {});

    // ---- gaozhi ----
    await queryInterface.removeIndex('gaozhi_submissions', 'idx_gaozhi_submissions_agg_qtype').catch(() => {});
    await queryInterface.addIndex('gaozhi_submissions', {
      name: 'uk_gaozhi_submission',
      unique: true,
      fields: ['aggregation_uid', 'question_uid', 'question_type'],
    }).catch(() => {});

    // ---- exam_submissions ----
    await queryInterface.removeIndex('exam_submissions', 'idx_exam_submissions_contestant_id').catch(() => {});
    await queryInterface.addIndex('exam_submissions', {
      name: 'uk_exam_submission_contestant',
      unique: true,
      fields: ['contestant_id'],
    }).catch(() => {});
  },

  down: async (queryInterface) => {
    await queryInterface.removeIndex('zhongzhi_submissions', 'uk_zhongzhi_submission').catch(() => {});
    await queryInterface.addIndex('zhongzhi_submissions', {
      name: 'idx_zhongzhi_submissions_agg_qtype',
      fields: ['aggregation_uid', 'question_uid', 'question_type'],
    });

    await queryInterface.removeIndex('gaozhi_submissions', 'uk_gaozhi_submission').catch(() => {});
    await queryInterface.addIndex('gaozhi_submissions', {
      name: 'idx_gaozhi_submissions_agg_qtype',
      fields: ['aggregation_uid', 'question_uid', 'question_type'],
    });

    await queryInterface.removeIndex('exam_submissions', 'uk_exam_submission_contestant').catch(() => {});
    await queryInterface.addIndex('exam_submissions', {
      name: 'idx_exam_submissions_contestant_id',
      fields: ['contestant_id'],
    });
  },
};