如何在 Node.js 中链接两个 Promise？

Question

我是 Node.js 的新手。我想链接两个承诺。当第一个 Promise 失败时，不应执行第二个 Promise。但是当第一个承诺失败时，我下面的代码总是执行第二个承诺。

function genQueryPromise(query, parameters) {
  var deferred = Q.defer();
  dbClient.executeAsPrepared(query, parameters,
    function(err) {
      if (err) {
        console.log(
            'error in query: ' + query + ", parameters: " + parameters);
        deferred.reject(new Error(err));
      } else {
        console.log("Query succeeded: " + query);
        deferred.resolve();
      }
    }
  );
  return deferred.promise;
}

var timeUuid = cql.types.timeuuid();
genQueryPromise(
    'INSERT INTO points_statement ' +
      '(user_id, statement_id, points_change, time_updated, note) ' +
    'VALUES (?, ?, ?, dateof(now()), ?)',
    ["mytest8",
     timeUuid,
     220,//{value: pointsChange, hint: cql.types.dataTypes.bigint},
     "test"]
  )
  .then(genQueryPromise(
    'UPDATE user_points SET points = points + ? WHERE user_id = ?',
    [{value: 220, hint: cql.types.dataTypes.bigint}, "mytest8"]
  ))
  .fail(function (error) {
    console.log("db error " + error);
  });

输出如下：

error in query: INSERT INTO points_statement (user_id, statement_id, points_change, time_updated, note) VALUES (?, ?, ?, dateof(now()), ?), parameters: mytest8,b5835850-2266-11e4-a871-f1a82b5a7753,220,test
db error Error: ResponseError: Expected 8 or 0 byte long (4)
Query succeeded: UPDATE user_points SET points = points + ? WHERE user_id = ?

你可以看到，即使第一个 promise 失败，第二个 promise 也被执行了。我做错了什么？谢谢。

Answer 1

您在构建承诺链时调用了genQueryPromise。在链中调用它：

genQueryPromise(
    'INSERT INTO points_statement ' +
      '(user_id, statement_id, points_change, time_updated, note) ' +
    'VALUES (?, ?, ?, dateof(now()), ?)',
    ["mytest8",
     timeUuid,
     220,//{value: pointsChange, hint: cql.types.dataTypes.bigint},
     "test"]
  )
  .then(function() {
    return genQueryPromise(
      'UPDATE user_points SET points = points + ? WHERE user_id = ?',
      [{value: 220, hint: cql.types.dataTypes.bigint}, "mytest8"]
    );
  })
  .fail(function (error) {
    console.log("db error " + error);
  });

添加一个函数包装器，以便在第一次调用完成后执行第二次调用，而不是同时执行。