使用 Promise 运行并发 firebase 请求

Question

我正在尝试使用 Promise.all() 同时执行多个 Firebase 云存储查询：

exports.onNewOrder = 
functions.database.ref('/orders/{orderId}').onCreate(event => {  
    const data = event.val();
    const orderedBy = data.ordered_by;
    const creations = data.creations;

    var promises = []
    for (var entry of creations.entries()) {
        var key = entry[0], value = entry[1];
        var creationPromise = admin.database().ref("creations/"+ key);
        creationPromise.once("value")
        .then((crSnapshot)=>{
            const crName = crSnapshot.val().name;
            const created = crSnapshot.val().created;
            return "crName+'\n'+created”;
        }).catch((error) => {
            console.log("creation details error : "+ error);
        });
        promises.push(creationPromise);
    }

    const userPromise = admin.database().ref("users/"+ orderedBy);
    userPromise.once("value")
    .then((snapshot) => {
        var name = snapshot.val().name;
        var email = snapshot.val().email;
        const userDetails = name+'\n'+email;
        return userDetails; 
    }).catch((error) => {
        console.log("user details error :"+ error);
    });

    promises.push(userPromise);

    return Promise.all(promises).then(results => {
        console.log("results: " + results)
        return true;
    }).catch((error) => {
        console.log("sendMail failed : "+ error);
    });
});

我希望看到每个已执行承诺的完整查询结果，但是我得到的是以下内容：

结果：

https://....firebaseio.com/creations/cr_1bfd5f16212f44c792a2cbe3e9c6e4cc
https://....firebaseio.com/creations/cr_343c363c2a214487ba8cb7101653f6af
https://....firebaseio.com/creations/cr_e3f7a0bafd1542ac898d96e86155b94e
https://....firebaseio.com/users/us_5e4c28e18e63454c861caa5c4fe6a6f0

我错过了什么吗？

Answer 1

以下应该可以解决问题。

详细here：

Promise.all(iterable) 方法返回一个 Promise 当 iterable 参数中的所有承诺都具有 已解决或当 iterable 参数不包含任何承诺时。它 以第一个拒绝的承诺的原因拒绝。

因此，您需要在promises 数组中添加一些承诺。

once() 方法确实返回了一个承诺，正如文档中的 here 所解释的那样。

exports.onNewOrder = 
functions.database.ref('/orders/{orderId}').onCreate(event => {  
    const data = event.val();
    const orderedBy = data.ordered_by;
    const creations = data.creations;

    const promises = []

    for (var entry of creations.entries()) {
        var key = entry[0];
        var creationPromise = admin.database().ref("creations/"+ key).once("value");
        promises.push(creationPromise);
    }

    const userPromise = admin.database().ref("users/"+ orderedBy).once("value");
    promises.push(userPromise);

    return Promise.all(promises)
    .then(results => {
        console.log("results: " + results);
        return true;
        //actually here you will probably do something else than just write the results to the console. 
        //In this case you should return a promise (instead of true).
    }).catch((error) => {
        console.log("sendMail failed : "+ error);
        return false;
    });

});