Parse.Cloud.afterSave 无法使用解析服务器

Question

我正在使用 parse.com 并编写了一个完美运行的云代码功能。当我转移到自托管解析服务器后端时，一些云代码功能停止工作。

Parse.Cloud.afterSave("League", function (request) {

    if (request.object.get("leaderboard") == null) {

        var leaderboard = Parse.Object.extend("Leaderboard");
        var newInstance = new leaderboard();

        newInstance.save(null , {useMasterKey: true})
            .then(function (result) {
                request.object.set("leaderboard", result);
                request.object.save(null ,{useMasterKey: true});
            },
            function (error) {
                console.log("Error");
            });
        });
    }else{
        var membersRelation = request.object.relation("members");
        var membersQuery = membersRelation.query();
        membersQuery.count(null , {useMasterKey: true})
            .then(function (totalNumber) {
                request.object.set("memberCount", totalNumber)
                request.object.save(null ,{useMasterKey: true});
            }, function (error) {
                console.log("Error")
            })
     }

如您所见，我为 League class 定义了 afterSave 挂钩。在我的钩子中，当我设置一个新值 (leaderboard and/or membersCount) 时，我必须再次更新同一个对象，所以保存不止一次被调用。

函数保存数据正常，但也造成死循环。我知道它发生是因为我调用 request.object.save() 将再次更改 League class 因此 afterSave 事件再次触发，依此类推。我不知道我该如何处理这种情况。有人建议我添加超时但不确定如何。你能帮忙解决这个问题吗？

谢谢

Answer 1

你的方法有两个问题：

leaderboard 上存在竞争条件。当第一次保存的承诺解决时，不会有 leaderboard，然后它会神奇地出现在那里 "at some point in the future"。最好在 beforeSave 中设置初始值，这样 league 的状态是已知且可预测的。
membersCount 上也存在竞争条件。想象一下，添加 and/or 删除 members 的两个更新同时进入。在您读取关系和写入计数之间，可能会发生其他更新。您最终可能会得到错误的计数，甚至是负数！

要解决 1，我们只需将 leaderboard 的创建移动到 beforeSave。为了解决 2，我们将 membersCount 的计算移至 beforeSave，使用提供的有关 member 加法和减法的脏对象信息，最后我们使用 increment 进行确保更新是原子的并避免竞争条件。

下面是带有单元测试的工作代码。请注意，如果我自己对此进行代码审查，我会 a) 想要测试添加多个成员和减去多个成员 b) 将第一个大测试分成多个测试，每个测试只测试一个东西。 c) 测试在同一个存档中添加和删除。

我正在使用 es6 结构因为我喜欢它们 ;)。

尝试添加很多评论，但如果有什么地方令人困惑，请随时问我。

PS 如果您不知道如何对您的云代码进行运行单元测试，请问另一个问题，因为它对于弄清楚这些东西是如何工作的（并查看在解析服务器单元测试中是你能找到的最好的文档）

祝你好运！

const addLeaderboard = function addLeaderboard(league) {
  // note the simplified object creation without using extends.
  return new Parse.Object('Leaderboard')
    // I was surprised to find that I had to save the new leaderboard
    // before saving the league. too bad & unit tests ftw.
    .save(null, { useMasterKey: true })
    // "fat arrow" function declaration.  If there's only a single
    // line in the function and you don't use {} then the result
    // of that line is the return value.  cool!
    .then(leaderboard => league.set('leaderboard', leaderboard));
}

const leagueBeforeSave = function leagueBeforeSave(request, response) {
  // Always prefer immutability to avoid bugs!
  const league = request.object;

  if (league.op('members')) {
    // Using a debugger to see what is available on the league
    // is super helpful, cause I have never seen this stuff
    // documented, but its obvious in a debugger.
    const membersAdded = league.op('members').relationsToAdd.length;
    const membersRemoved = league.op('members').relationsToRemove.length;
    const membersChange = membersAdded - membersRemoved;
    if (membersChange !== 0) {
      // by setting increment when the save is done, the
      // change in this value will be atomic.  By using a change
      // in the value rather than an absolute number
      // we avoid a race condition when paired with the atomicity of increment
      league.increment('membersCount', membersChange);
    }
  }

  if (!league.get('leaderboard')) {
    // notice we don't have to save the league, we just
    // add the leaderboard.  When we call success, the league
    // will be saved and the leaderboard will be there....
    addLeaderboard(league)
      .then(() => response.success(league))
      .catch(response.error);
  } else {
    response.success(league);
  }
};

// The rest of this is just to test our beforeSave hook.
describe('league save logic', () => {

  beforeEach(() => {
    Parse.Cloud.beforeSave('League', leagueBeforeSave);
  });

  it('should create league and increment properly', (done) => {
    Parse.Promise.when([
      new Parse.Object('Member').save(),
      new Parse.Object('Member').save(),
      new Parse.Object('Member').save(),
      new Parse.Object('Member').save(),
    ])
      .then((members) => {
        const league = new Parse.Object('League');
        const memberRelation = league.relation('members');
        memberRelation.add(members);
        // I want to use members in the next promise block,
        // there are a number of ways to do this, but I like
        // passing the value this way.  See Parse.Promise.when
        // doc if this is mysterious.
        return Parse.Promise.when(
          league.save(null, { useMasterKey: true }),
          members);
      })
      .then((league, members) => {
        expect(league.get('leaderboard').className).toBe('Leaderboard');
        expect(league.get('membersCount')).toBe(4);
        const memberRelation = league.relation('members');
        memberRelation.remove(members[0]);
        return league.save(null, { useMasterKey: true });
      })
      .then((league) => {
        expect(league.get('membersCount')).toBe(3);
        // just do a save with no change to members to make sure
        // we don't have something that breaks in that case...
        return league
          .set('foo', 'bar')
          .save(null, { useMasterKey: true })
      })
      .then(league => {
        expect(league.get('foo')).toBe('bar');
        done();
      })
      .catch(done.fail);
  });

  it('should work to create new without any members too', (done) => {
    new Parse.Object('League')
      .save() // we don't really need the useMasterKey in unit tests unless we setup `acl`s..:).
      .then((league) => {
        expect(league.get('leaderboard').className).toBe('Leaderboard');
        done();
      })
      .catch(done.fail);
  });
});

Parse.Cloud.afterSave 无法使用解析服务器

Parse.Cloud.afterSave not working with parse-server

javascript

parse-platform

parse-server

parse-cloud-code