Bluebird Promise serial iteration, and resolve to

问题:

I have this promise that creates a new Item document if it's not found in the db, and then stores it in a previously created Collection document..

The Collection document is the first string in an array, and any subsequent index in the array translates to one or more Item docs.

Promise.each "Resolves to the original array unmodified" and so the last return within the Promise.each is rendering the objects, but the subsequent .then produces the original array..

Here's the promise (abbreviated for readability):

globalVar = true;
collectionId = "";
var itemSeries = Promise.each(items, function(element) {
    if (globalVar == true) {
        return Models.Collection.findOneAsync({
            "name": element
        })
        .then(function(collection) {
            // promise chain similar to the following else..
            // set the collectionId var to an _id
        });
    } else {
        return Models.Items.findOneAsync({
            "name": element
        })
        .then(function(item) {
            if (item == null) {
                return Models.Labels.findOneAsync({
                    "title": element
                })
                .then(function(label) {
                    var newItem = new Models.Items({
                        name: element,
                        label: label._id
                    });
                    return newItem.saveAsync();
                }).then(function() {
                    return Models.Items.findOneAsync({
                        "name": element
                    });
                }).then(function(item) {
                    item.collection = collectionId;
                    return item.saveAsync();
                }).then(function() {
                    return Models.Items.findOneAsync({
                        "name": element
                    });
                }).then(function(item) {
                    allItems.push(item);
                    console.log("allItems: [ ");
                    console.log(allItems);
                    return allItems;
                });
            }
        });
    }
}).then(function(allItems) {
    console.log("allItems: [ ");
    console.log(allItems);
    return allItems;
});

And here's the last of the console.log within the Promise.each:

allItems: [ 
[ { _id: 54eec5f2b9fb280000286d52,
    name: 'one',
    label: 54eec5f2b9fb280000286d51,
    collection: 54eec5f2b9fb280000286d50,
    __v: 0 },
  { _id: 54eec5f2b9fb280000286d54,
    name: 'two',
    label: 54eec5f2b9fb280000286d53,
    collection: 54eec5f2b9fb280000286d50,
    __v: 0 } ]

And then after the subsequent .then(function(allItems) { here's the last console.log:

allItems: [ 
[ 'collectionName', 'one', 'two' ]

Also, the variable itemSeries that = Promise.each later renders undefined in a Promise.join?

回答1:

The .each function will not change the value that is passed through the chain:

I simplified your code, as input I assume:

var items = ['one','two'];

For your code:

Promise.each(items, function(element) {
    return element+'.';
    //return Promise.resolve(element+'.');
})
.then(function(allItems) {
    console.dir(allItems);
});

The result will still be ['one','two'] because this are resolved values of the array items. The returned value within the each does not influence the content of the value passed to the chained then.

The .map function on the other hand will have this effect:

Promise.map(items, function(element) {
    return element+'.';
    //return Promise.resolve(element+'.');
})
.then(function(allItems) {
    console.dir(allItems);
});

Here the return value value will be used to create a new array which will then be passed to the then. Here the result would be ['one.','two.'].

The two allItems appearing in your code are different objects.

EDIT For serially iteration with mapping I would write a helper function like this:

 function mapSeries(things, fn) {
     var results = [];
     return Promise.each(things, function(value, index, length) {
         var ret = fn(value, index, length);
         results.push(ret);
         return ret;
     }).thenReturn(results).all();
 }

Source: Implement Promise.series

回答2:

Bluebird now natively implements a mapSeries, see http://bluebirdjs.com/docs/api/promise.mapseries.html

It also looks like Promise.each still returns the original array unfortunately in v3.x.x.