Inside a service, I would like to load a resource using $http. Once loaded resource, I want to store it in a variable. Then, I need to load a child resource and store it too. I know that the promise is designed for this kind of work, but there seems to be so much how to use it I get a little confusion. Here is my code:
var project = {};
var todo = {};
function init(){
var idProject = 21;
var idTodo = 6;
// If idProject is specified
if ( idProject != null ) {
// First, load project data
var promise = WorkspaceManager.getProject($rootScope.workspace, idProject);
// Then save project data
promise.then(function(response){
project = response.data;
return project;
});
if ( idTodo != null ) {
// Then load todo data
promise.then(function(project){
return ProjectManager.getTodo(project, idTodo);
});
// Then save todo data
promise.then(function(response){
todo = response.data;
return todo;
});
}
}
console.log(project); // returns {}
}
init()
Thanks in advance !
If I understand correctly, this is trickier than it appears at first glance. You appear to need a function that chains two asynchronous processes and returns a promise of a composite value comprising data acquired by the first and the second processes.
At least two approaches are available :
Easy but inelegant: In each asynchronous process, accumulate the required values as properties of an outer object.
Elegant but awkward: From each asynchronous process, accumulate the required values as properties of an object, a promise of which is returned.
The code below adopts the first approach :
function init() {
var idProject = 21;
var idTodo = 6;
var projectObj = {};//This object acts as a "bank" for asynchrounously acquired data.
if ( idProject != null ) {
return WorkspaceManager.getProject($rootScope.workspace, idProject).then(function(response) {
projectObj.project = response.data;//put `response.data` in the bank as .project.
if( idTodo != null ) {
return ProjectManager.getTodo(response.data, idTodo);
}
}).then(function(toDo) {
projectObj.toDo = toDo;//put `toDo` in the bank as .toDo .
return projectObj;
});
}
}
init().then(function(projectObj) {
console.log(projectObj.project);
console.log(projectObj.toDo);
});
Or (still the first approach) with error handlers :
function init() {
var idProject = 21;
var idTodo = 6;
var projectObj = {};//This object acts as a "bank" for asynchrounously acquired data.
if ( idProject != null ) {
return WorkspaceManager.getProject($rootScope.workspace, idProject).then(function(response) {
projectObj.project = response.data;//put `response.data` in the bank as .project.
if( idTodo != null ) {
return ProjectManager.getTodo(response.data, idTodo);
}
else {
$q.defer().reject('idTodo invalid');
}
}).then(function(toDo) {
projectObj.toDo = toDo;//put `toDo` in the bank as .toDo .
return projectObj;
});
}
else {
return $q.defer().reject('idProject invalid');
}
}
init().then(function(projectObj) {
console.log(projectObj.project);
console.log(projectObj.toDo);
}, function(errorMessage) {
console.log(errorMessage);
});
untested
The way you doing, you're creating "brothers" promise derived from the first promise. All the promises are going to be resolved as soon as WorkspaceManager.getProject promise has been resolved. What I believe you want is to chain them all, in way that when first promise gets resolved, yo asks for Todo data, when you got it, you asks to save it. If this is the case, you shall grab the derived promise from each promise.
// Then save project data
promise = promise.then(function(response){
project = response.data;
return project;
});
// Then load todo data
promise = promise.then(function(project){
return ProjectManager.getTodo(project, idTodo);
});
// Then save todo data
promise.then(function(response){
todo = response.data;
return todo;
});
Trying to illustrate a bit more, the first approach is like:
var mainPromise = ...;
mainPromise.then(function loadTodo(mainPromiseReturn){});
mainPromise.then(function saveTodo(mainPromiseReturn){});
The loadTodo and saveTodo are pararell, they're not chained to each other. They both receive the same data.
The approach I suggest is like:
var mainPromise = ...;
mainPromise
.then(function loadTodo(mainPromiseReturn){})
.then(function saveTodo(loadTodoReturn){});
Related
Still trying to wrap my head around promises and how they work. Im querying the Google webmaster API to return Search Analytics data. I've set up a promise which returns the data if i call it once however i need to call it again based on the result of the previous.
For example:
startRow = 0;
data = [];
Query(startRow).then((results) => {
if (results != null) {
data.push(results)
startRow++;
// RUN SAME QUERY AGAIN
};
});
startRow needs to increase by 1 then call the same promise (with the updated startRow) if the promise returned data. Is this possible or am i looking at this totally the wrong way?
You can't call the same Promise more than once, only create new ones.
startRow = 0;
data = [];
function startQuery() {
// Generally a good idea to always return Promises,
// so you can chain them if needed
return Query(startRow).then(processResults);
}
function processResults(results) {
if (results == null) return;
data.push(results);
startRow++;
return startQuery();
};
startQuery();
Or, in a more compact way:
startRow = 0;
data = [];
function startQuery() {
return Query(startRow).then((results) => {
if (results == null) return;
data.push(results);
startRow++;
return startQuery();
});
}
startQuery();
What you can do is create a function that recursively returns all of the results from a certain page onward, then call that with an initial page value of 0:
function queryPaged(pageNum, soFar) {
return Query(pageNum).then(function (results) {
return results
? queryPaged(pageNum + 1, soFar.concat(results))
: soFar;
});
}
queryPaged(0, []).then(function (allResults) {
console.log(allResults);
});
I am trying to fill an array with records from a mongoDB database using mongoose. When I am trying to fill the records. It shows an empty array outside the function even though I am declaring the outside the function. Below is the code.
var saveMessageToVariable = function(){
var records = [];
var spark_ids = [];
var obj = new Object();
Message.find().distinct("spark_id").exec(function(err,data) {
data.forEach(function (id) {
if(id != null)
spark_ids.push(id);
});
// console.log(spark_ids.length);
spark_ids.forEach(function(spark_id){
Message.findOne({"spark_id":spark_id}).sort({"date":-1}).exec(function(err,data){
obj.spark_id = data.spark_id;
obj.meesage = data.message;
obj.date = data.date;
obj.message_id = data._id;
records.push(obj);
});
});
});
console.log(records);
}
When I run this, the log is showing an empty array. How do I resolve this issue?
It's an asynchronous call and as soon as data is fetched from database control shifts to next line and therefore prints the initial value, I would prefer you to use a callback like this:
function(spark_id,callback){
Message.findOne({"spark_id":spark_id}).sort({"date":-1}).exec(function(err,data){
obj.spark_id = data.spark_id;
obj.meesage = data.message;
obj.date = data.date;
obj.message_id = data._id;
callback(obj);
});
}
function(obj)
{
records.push(obj);
}
You two other approachs for this:
1) use try and catch block.
2) use async and await keyword.
Cheers!
I don't have much experience with moongoose, but according to the docs it supports promises since Version 4.
Then this should work:
//I assume you'll need this more often
function notNull(value){ return value != null; }
//returns a promise of the records-Array
var saveMessageToVariable = function(){
//returns a promise of a formated message
function getMessage( spark_id ){
return Message.findOne({ spark_id })
.sort({ date: -1 })
//.exec()
.then( formatMessage )
}
function formatMessage( msg ){
return {
spark_id: msg.spark_id,
message: msg.message,
date: msg.date,
message_id: msg._id
}
}
return Message.find()
.distinct("spark_id")
//.exec()
.then(function( ids ){
//waits for all findOnes to complete, then returns an Array
return Promise.all(
ids.filter( notNull ).map( getMessage )
));
}
I'm not sure, wether you need exec() in this code or not. You should check that.
//usage
saveMessageToVariable.then(function(records){
console.log(records);
})
btw. saveMessageToVariable doesn't reflect at all what this function does. You should choose a better name.
I am implementing this scenario where I have to fetch data from multiple URLs iteratively and process it with some business logic and display on screen. I am implementing this in the controller as it is a requirement. All is well until part-1 and I am getting the 6 promise objects in the promises array. But, I am not getting the data into metricData. I am seeing a null in the console while running in the browser. I am sure that the data is coming in the URL response. I feel I am doing something silly in the $q.all method. Is this correct?
var calculateMutationsInDepth = function(){
//Part-1
var promises=[];
var metricData=[];
for(var depth=0 ; depth<6 ; depth++){
var resourceUrl = urlService(depth);
promises.push($http.get(resourceUrl)
.then(function(response){
return response.data;
},function(status){
return status;
}));
}
//Part-2 Resolving the promise array below
$q.all(promises).then(function(data){
for(var eachResult=0; eachResult < data.length; eachResult++){
if(null != data[eachResult]){
var eachDataObject = data[eachResult];
//For debugging console.log(eachDataObject);
for(var objCount=0; objCount < eachDataObject.length; objCount++){
if(eachDataObject[objCount].scope === "PRJ" || eachDataObject[objCount].scope === "FIL")
metricData.push(eachDataObject[objCount]);
}
}
}
});
if(metricData != null){
analyzeMutationData(metricData); //Calling a function with the aggregated data array where business logic is present
}
};
calculateMutationsInDepth(); //Calling the above function
Yes, something silly.
As written, analyzeMutationData(metricData) is called synchronously whereas metricData is populated asynchronously inside the $q.all(promises).then() callback.
Also, as written the error handler function(status){ return status; } is inappropriate. Either :
omit the error handler entirely and allow any single $http error to prevent further processing in Part 2, or
return null, allowing processing in Part 2, and the if(dataObject != null) test in part 2 to filter out any such error.
Here's the revised code with a few other tidies and a demonstration of what can be done if calculateMutationsInDepth() returns a promise.
var calculateMutationsInDepth = function() {
//Part-1
var depth, promises = [];
for(depth=0; depth<6; depth++) {
promises.push($http.get(urlService(depth))
.then(function(response) {
return response.data;
}, function(error) {
return null; // error recovery - `dataObject` below will be null
}));
}
//Part-2 Aggregate the promises, extract metric data and apply business logic
return $q.all(promises).then(function(data) { // note `return` here
var dataObject, i, j, metricData = [];
for(i=0; i<data.length; i++) {
dataObject = data[i];
if(dataObject != null) {
for(j=0; j<dataObject.length; j++) {
if(dataObject[j].scope === "PRJ" || dataObject[j].scope === "FIL") {
metricData.push(dataObject[j]);
}
}
}
}
// Analyse here, inside the .then()
if(metricData.length > 0) { // metricData is an array and will never be null, therefore test metricData.length.
return analyzeMutationData(metricData);
}
return null;
});
};
calculateMutationsInDepth().then(function(analysis) {
// all complete
// `analysis` is either null or whatever `analyzeMutationData(metricData)` returned.
}).catch(function(error) {
console.log(error);
});
Hope this helps you out! Let me know if it doesn't.
I know this is wrong, but essentially I want to
connect to a db/orm via a promise
wait on that promise to fulfill and get the models (the return from the promise)
use the results for form a middleware generator function to place the models on the request
I suspect that this isn't the best approach, so essentially I have two questions:
Should I be rewriting my db/orm connect to a generator function (I have a feeling that is more inline with koa style)
Back to the original question (as I am sure I will not get a chance to rewrite all my business logic) - how do I wait on a promise to fulfill and to then return a generator function?
This is my poor attempt - which is not working, and honestly I didn't expect it to, but I wanted to start by writing code, to have something to work with to figure this out:
var connectImpl = function() {
var qPromise = q.nbind(object.method, object);
return qPromise ;
}
var domainMiddlewareImpl = function() {
let connectPromise = connectImpl()
return connectPromise.then(function(models){
return function *(next){
this.request.models = models ;
}
})
}
var app = koa()
app.use(domainMiddlewareImpl())
According to this, you can do the following:
var domainMiddlewareImpl = function() {
return function *(){
this.request.models = yield connectImpl();
};
};
A context sensitive answer based on the info provided by Hugo (thx):
var connectImpl = function() {
var qPromise = q.nbind(object.method, object);
return qPromise ;
}
var domainMiddlewareImpl = function () {
let models = null ;
return function *(next) {
if(models == null){
//////////////////////////////////////////////////////
// we only want one "connection", if that sets up a
// pool so be it
//////////////////////////////////////////////////////
models = yield connectImpl() ;
}
this.request.models = models.collections;
this.request.connections = models.connections;
yield next
};
};
My example the connectImpl is setting up domain models in an ORM (waterline for now), connecting to a database (pooled), and returning a promise for the ORM models and DB connections. I only want that to happen once, and then for every request through my Koa middleware, add the objects to the request.
Say we have a Customer Object, that has a "Foo" collection. I'd like my "getCustomer" function to add all Foos it does not have already, and then return itself, as a promise...
So I'd like a promise to: Get a Customer, then add all missing Foos to this Customer, so that when the promise is resolved, the customer has all missing foos.
Example:
// dataservice.js
// returns Q.Promise<breeze.QueryResult>
function getCustomer(custId) {
var query = breeze.EntityQuery().from("Customers").where("CustomerId", "==", custId);
return this.em.executeQuery(query);
}
// returns Q.Promise<breeze.QueryResult>
function getFoosNotOnCustomer(customer) {
var query = breeze.EntityQuery().from("Foo").where("CustomerId", "!=", customer.Id());
return this.em.executeQuery(query);
}
I am struggling with how to "chain" these together properly, what do do if no customer is found, etc. How can I modify "getCustomer" to do this? I am basically trying to user breeze synchronously. Here is my attempt, but it turns into ugly nested code in a hurry.
// want to return Q.Promise<Customer> that has Foos loaded
// I think this is actually returning something like Q.Promise<Q.Promise<Customer>>
function getCustomer(custId) {
var query = breeze.EntityQuery().from("Customers")
.where("CustomerId", "==", custId);
return this.em.executeQuery(query) // return here?
.then(function(data) {
// what about conditionals?
if(data.results.length == 1) {
getFoosNotOnCustomer(data.results[0]).
then(function (foosArray) {
$.each(foosArray, function(i,el) {
// push foos onto customer instance
}
return custWithFoos; // return here?
}
// return something?
}
}
}
Here's what I ended up doing:
function getCustomer(custId) {
var query = breeze.EntityQuery().from("Customers").where("CustomerId", "==", custId);
return manager.executeQuery(query) // return here?
.then(addFoos)
.then(doSomethingElse);
}
function addFoos(data) {
var myDefer = Q.Defer();
if (data && data.result.length == 1) {
var customer = data.results[0];
var query = // get FOOS Customer doesn't have;
manager.executeQuery(query).then(function (fooData) {
$.each(fooData.results function (i, el) {
customer.Foos.push(el);
});
myDefer.reslove(customer);
});
} else {
myDefer.resolve(undefined);
}
return myDefer.promise;
}
function doSomethingElse(customer) {
var myDefer = Q.Defer();
customer.SomePropert("test");
return myDefer.resovlve(customer);
}
// ----- MVVM
var custPromise = getCustomer(1).then(function (customer) {
// do something
});
I will take your example on face value despite my inability to understand the semantics ... in particular my inability to understand why getting all Foos not belonging to the customer is going to be helpful.
I'll just focus on "chaining" and I'll assume you want the caller to take possession of the selected customer when you're done.
Sequential chaining
In this example, we wait for the customer before getting the Foos
function getCustomer(custId) {
var cust;
var em = this.em;
var query = breeze.EntityQuery().from("Customers")
.where("CustomerId", "==", custId);
// On success calls `gotCustomer` which itself returns a promise
return em.executeQuery(query)
.then(gotCustomer)
.fail(handleFail); // you should handleFail
// Called after retrieving the customer.
// returns a new promise that the original caller will wait for.
// Defined as a nested success function
// so it can have access to the captured `cust` variable
function gotCustomer(data) {
cust = data.results[0];
if (!cust) {
return null; // no customer with that id; bail out now
}
// got a customer so look for non-customer foos
// returning another promise so caller will wait
return breeze.EntityQuery().from("Foos")
.where("CustomerId", "!=", custId)
.using(em).execute()
.then(gotFoos);
}
// Now you have both the customer and the other Foos;
// bring them together and return the customer.
function gotFoos(data) {
var foos = data.results;
// assume `notMyFoos` is an unmapped property that
// should hold every Foo that doesn't belong to this Customer
foos.forEach(function(f) { cust.notMyFoos.push(f); }
return cust; // return the customer to the caller after Foos arrive.
}
}
Parallel async queries
In your scenario you really don't have to wait for the customer query before getting the foos. You know the selection criterion for both the customer and the foos from the start. Assuming you think that there is a high probability that the customer query will return a customer, you might fire off both queries in parallel and then mash up the data when both queries complete. Consider the Q.all for this.
function getCustomer(custId) {
var em = this.em;
var custPromise = breeze.EntityQuery().from("Customers")
.where("CustomerId", "==", custId)
.using(em).execute();
var fooPromise = breeze.EntityQuery().from("Foos")
.where("CustomerId", "!=", custId)
.using(em).execute();
Q.all([custPromise, fooPromise])
.then(success)
.fail(handleFail); // you should handleFail
// Now you have both the customer and the "other" Foos;
// bring them together and return the customer.
// `data` is an array of the results from each promise in the order requested.
function success(data) {
var cust = data[0].results[0];
if (!cust) return null;
var foos = data[1].results;
// assume `notMyFoos` is an unmapped property that
// should hold every Foo that doesn't belong to this Customer
foos.forEach(function(f) { cust.notMyFoos.push(f); }
return cust; // return the customer to the caller after Foos arrive.
}
}
Notice that I don't have to do so much null checking in the success paths. I'm guaranteed to have data.results when the success callback is called. I do have to account for the possibility that there is no Customer with custId.