如果找不到结果，如何停止重新思考数据库调用？

Question

I 运行 RethinkDB 命令与 Node.js (babel) 异步调用：

let user = await r.table('users').filter({key: key}).limit(1).run();

如果数据库找不到结果，如何停止异步调用？

Answer 1

使用await函数意味着节点将等待异步r.table(...命令到return，然后再继续下一行代码，这意味着它的行为在逻辑上就好像它是同步代码。

当 RethinkDB 找到具有指定键的第一个 'user' 文档时，您的特定命令应该 return。如果找不到结果，则无需 "stop"，它会在 (a) 找到结果或 (b) 完成扫描整个 table.

后立即停止

通常 "stopping" node/javascript 中的异步代码是不可能的，但您可以限制等待异步方法的时间。这是使用 Promise.race() 函数的示例。

/*
 * toy async function
 *
 * returns a promise that resolves to the specified number `n` 
 * after the specified number of seconds `s` (default 2)
 */
const later = (n, s=2) => {
    return new Promise(resolve => {
        setTimeout(() => resolve(n), s*1000);
    })
}

/*
 * returns a promise that rejects with `TIMEOUT_ERROR` after the 
 * specified number of seconds `s` 
 */
const timeout = (s) => {
    return new Promise((resolve, reject) => {
        setTimeout(() => reject("TIMEOUT_ERROR"), s*1000)
    })
}

/* 
 * Example 1: later finished before timeout
 * later resolves after 1 second, timeout function rejects after 3 seconds
 * so we end up in the `.then` block with `val == 42`
 */
Promise.race([later(42, 1), timeout(3)])
    .then(val => {
        // do somethign with val...
        console.log(val)
    }).catch(err => {
        if (err === "TIMEOUT_ERROR") {
            console.log("we timed out!")
        } else {
            consle.log("something failed (but it was not a timeout)")
        }
    });

/* 
 * Example 2 (using async/await syntax): we timeout before later returns.
 * later resolves after 3 seconds, timeout function rejects after 2 seconds
 * so we end up in the `.catch` block with `err == "TIMEOUT_ERROR"`
 */
try {
    const val = await Promise.race([later(11, 3), timeout(2)]);
    // do something with val...
} catch (err) {
    if (err === "TIMEOUT_ERROR") {
        console.error("we timed out!")
    } else {
        console.error("something failed (but it was not a timeout)")
    }
}

如果找不到结果，如何停止重新思考数据库调用？

How to stop rethink database call if cannot find result?

asynchronous

node.js

rethinkdb