q.js demo for having an arbitrary number of sub-promises generated from an initial promise

map_reduce_subquery_breaks.js

// @cpdean
// demonstrates how to have one promise kick off
// an additional arbitrary number of promises, and
// then merge their results down again after
// all the promises are complete.
var Q = require('q'); // "q": "~1.0.0"


// initial query that generates seed data for more work to be done
function fakeQuery(){
    var deferred = Q.defer();
    setTimeout(function(){
        console.log("first query complete");
        deferred.resolve([{id: 1}, {id: 0}, {id: 3}]);
    }, 1e3);
    return deferred.promise;
}

// second query
function deferredPropertyLookup(x) {
    var deferred = Q.defer();
    setTimeout(function(){
        console.log("second query starting " + x); 
        if(x != 0){
            deferred.resolve(x);
        }
        else{
            deferred.reject(new Error("such pains"));
        }
    }, 1e3);
    return deferred.promise;
}

fakeQuery().then(function(results){
    // `results` could be an arbitrary number of things
    // to do more work on

    // then concurrently run the lookup function with its
    // post processing step in then()
    var with_properties = results.map(function(e){
        return deferredPropertyLookup(e.id).then(function(found){
            return {id: e.id, prop: found};
        }); 
    }); 


    return Q.all(with_properties).done(function(a){
        // print the results when the lookups and processing are done
        // todo:  is it possible to salvage the sub promises that did manage
        //        to resolve?
        console.log(a);
    }, function(error){
        console.log("Everything died!");
        console.log(error.stack);
    }); 
});

package.json

{
  "name": "mapreduce-bug",
  "version": "0.0.0",
  "description": "",
  "main": "map_reduce.js",
  "scripts": {
    "test": "echo \"Error: no test specified\" && exit 1"
  },
  "repository": {
    "type": "git",
    "url": " "
  },
  "author": "",
  "license": "BSD",
  "dependencies": {
    "q": "1.0.0"
  }
}