I got a MySQL query where I ask for the values in a specific field with the help of the SELECT statement. My question is: How do I get the value of the query ? When I render the template with my current code I receive following output on my page [object Object]. I have the following Code:
var network;
function query(sql, callback) {
connection.query(sql, function(err, rows) {
if (err) {
callback(err, null);
} else {
callback(rows);
res.render('index.hjs', { Mysql : network });
console.log(network);
}
});
}
query("SELECT testString FROM test", function(results){
network = results;
});
There are a couple of corrections in your code. Nothing wrong but just good practice. In error callback, you are sending two arguments but in actual query callback function, you are using only one argument.
Second thing is that res will be undefined inside query function. (Of course, unless you have it inside request handler, which spoils the purpose of having it as a function)
Without your database structure and code structure, this is the best I can do to help you.
Related
router.post("/application_action", function(req,res){
var Employee = req.body.Employee;
var conn = new jsforce.Connection({
oauth2 : salesforce_credential.oauth2
});
var username = salesforce_credential.username;
var password = salesforce_credential.password;
conn.login(username, password, function(err, userInfo, next) {
if (err) { return console.error(err); res.json(false);}
// I want this conn.query to execute first and then conn.sobject
conn.query("SELECT id FROM SFDC_Employee__c WHERE Auth0_Id__c = '" + req.user.id + "'" , function(err, result) {
if (err) { return console.error(err); }
Employee["Id"] = result.records[0].Id;
});
//I want this to execute after the execution of above query i.e. conn.query
conn.sobject("SFDC_Emp__c").update(Employee, function(err, ret) {
if (err || !ret.success) { return console.error(err, ret);}
console.log('Updated Successfully : ' + ret.id);
});
});
I have provided my code above. I need to modify Employee in the conn.query and use it in conn.sobject. I need to make sure that my first query executes before 2nd because I am getting value from 1st and using in the 2nd. Please do let me know if you know how to accomplish this.
New Answer Based on Edit to Question
To execute one query based on the results of the other, you put the second query inside the completion callback of the first like this:
router.post("/application_action", function (req, res) {
var Employee = req.body.Employee;
var conn = new jsforce.Connection({
oauth2: salesforce_credential.oauth2
});
var username = salesforce_credential.username;
var password = salesforce_credential.password;
conn.login(username, password, function (err, userInfo, next) {
if (err) {
return console.error(err);
res.json(false);
}
// I want this conn.query to execute first and then conn.sobject
conn.query("SELECT id FROM SFDC_Employee__c WHERE Auth0_Id__c = '" + req.user.id + "'", function (err, result) {
if (err) {
return console.error(err);
}
Employee["Id"] = result.records[0].Id;
//I want this to execute after the execution of above query i.e. conn.query
conn.sobject("SFDC_Emp__c").update(Employee, function (err, ret) {
if (err || !ret.success) {
return console.error(err, ret);
}
console.log('Updated Successfully : ' + ret.id);
});
});
});
});
The only place that the first query results are valid is inside that callback because otherwise, you have no way of knowing when those asynchronous results are actually available and valid.
Please note that your error handling is unfinished since you don't finish the response in any of the error conditions and even in the success case, you have not yet actually sent a response to finish the request.
Original Answer
First off, your code shows a route handler, not middleware. So, if you really intend to ask about middleware, you will have to show your actual middleware. Middleware that does not end the request needs to declare next as an argument and then call it when it is done with it's processing. That's how processing continues after the middleware.
Secondly, your console.log() statements are all going to show undefined because they execute BEFORE the conn.query() callback that contains the code that sets those variables.
conn.query() is an asynchronous operation. It calls its callback sometime IN THE FUTURE. Meanwhile, your console.log() statements execute immediately.
You can see the results of the console.log() by putting the statements inside the conn.query() callback, but that is probably only part of your problem. If you explain what you're really trying to accomplish, then we could probably help with a complete solution. Right now, you're just asking questions about flawed code, but not explaining the higher level problem you're trying to solve so you're making it hard for us to give you the best answer to your actual problem.
FYI:
app.locals - properties scoped to your app, available to all request handlers.
res.locals - properties scoped to a specific request, available only to middleware or request handlers involved in processing this specific request/response.
req.locals - I can't find any documentation on this in Express or HTTP module. There is discussion of this as basically serving the same purpose as res.locals, though it is not documented.
Other relevants answers:
req.locals vs. res.locals vs. res.data vs. req.data vs. app.locals in Express middleware
Express.js: app.locals vs req.locals vs req.session
You miss the basics of the asynchronous flow in javascript. All the callbacks are set to the end of event loop, so the callback of the conn.query will be executed after console.logs from the outside. Here is a good article where the the basic concepts of asynchronous programming in JavaScript are explained.
I have array of events, for each value there may/may not query fires to get data.
var eventTypes = [["write","write","write","write"],["write","write","read","write"]];
_.each(eventTypes,function(obj){
gettime(obj);
});
gettime=function(events){
var resultArray = [];
_.each(events,function(event){
if(event === "read"){
resultArray.push(makesqlquery(event));
}else{
resultArray.push({"time":current_time})
}
});
q.all(resultArray).then(function(finalResult){
insertIntoPostgreSQL(finalResult);
});
}
makesqlquery = function(event){
var deferred = q.defer();
sql.query("select time from events where eventtype ="+ event,
function(result,error){
if(error){
deferred.reject(error);
}else{
deferred.resolve({time:result.time});
}
});
return deferred.promise;
}
In the above code I'm able to push 1st set of data(["write","write","write","write"]) into postgresql database but not 2nd set(["write","write","read","write"]). Whenever I get read event in a set, I'm getting empty object. What would be the problem? For the above example I should have 8 records in postgresql, but I see only first array's four data.
More Info: insertIntoPostgreSQL() function will get list of objects and insert each object into database. This operation is workin fine.
I tried use two console stmt as
console.log("sql result:"result);
deferred.resolve({time:result.time});
and
console.log("Before Insert:"JSON.stringigy(resultArray));
q.all(resultArray).then(function(finalResult){
I get result as in the following order.
Before insert:[{"source":{}},{"source":{}},{"source":{}},{"source":{}}]
Before insert:[{}]
sql result:{time:"2015-07-10 00:00:00"}
As Roamer-1888 mentioned in the comments, definitely add an error handler for your Q.all. The basic structure of your promise seems to be fine, the error is somewhere else.
It looks like the result in your sql.query callback is not quite what you expect, as it is read as undefined. Because of an error there, Q.all is not getting resolved, therefore nothing gets added to your database.
var userLat = db.collection('users', function (err, document){
document.findOne({_id: loggedUserID}, function(err, docs) {
console.log(docs.currentUserLat);
})
});
This is my code, I'm trying to get the value that's console logged into the variable. I just can't find the correct syntax to do this. The console log does return the correct value just need to drop it into the variable. Grateful for some help.
What do you want to do with 'docs.currentUserLat'?
You can do what you need to do without saving docs.currentUserLat to a variable that has scope outside of your db.collection call. Some examples:
If you simply want to change the document in your database, take advantage of the many methods specified in the Collections API: http://mongodb.github.io/node-mongodb-native/2.0/api/Collection.html. For example, to update the document and simultaneously resave it in the database:
db.collection('users', function (err, document){
document.findOneAndUpdate({_id: loggedUserID},
{currentUserLat: [updated value]},
function(err, docs) {
if(err) console.log(err);
}
)
});
If you just wanted to use docs.currentUserLat inside some node function, you'll need to properly nest the document.findOne function inside a callback (or vice versa). For example, to write currentUserLat to a file using the fs module:
var fs = require('fs');
db.collection('users', function (err, document){
document.findOne({_id: loggedUserID}, function(err, docs) {
fs.writeFile("pathToYourFile", docs.currentUserLat, function(err) {
if(err) {return console.log(err);}
});
});
});
Or, if you want to send it in response to a simple http request:
var http = require('http');
http.createServer(function(request,response){
db.collection('users', function (err, document){
document.findOne({_id: loggedUserID}, function(err, docs) {
response.writeHead(200,{'Content-Type':'text/html'});
response.end(docs.currentUserLat);
});
});
});
The key thing to remember is what JohnnyHK said in their comment: docs.currentUserLat is only available inside the anonymous function passed to findOne. So, whatever it is that you need to do, do it inside this function.
(Reading the link JohnnyHK provided is a great way to get started with understanding asynchronous functions in Node. Another is https://github.com/rvagg/learnyounode)
First of all you have to understand how javascript callback works. After that you will see that nothing assigns docs.currentUserLat to your userLat variable. The reason behind this is that your docs.currentUserLat is available only inside the callback. Think about it in the following way:
You program started to execute and encountered the line: var userLat = .... This line tells: do a callback (which basically asks someone else to do the job), your while your job is being executed the program continues, by assigning userLat to undefined and executes further. Then at some period of time callback finishes and console.log your docs.currentUserLat.
One way to have the desired behavior is to make userLat global and instead of console.log(docs.currentUserLat); do userLat = docs.currentUserLat. The problem that if you will do this, your userLat eventually will have the desired value (if callback will not fail), but you can not predict when. So if you will do
var userLat = db.collection('users', function (err, document){ ... });
.. some other code
console.log(userLat);
you will not be sure that you will get the output. Another way to do put everything in another callback.
function searchCoords(){
var result = result;
connection.query('SELECT * FROM monitoring', function(err, result){
if(err){
console.log(err);
}
return{
result: result};
});
}
That's my code. I'm using that code to find the last coordinates of some devices and display them in google maps. but i need to first be able to access the array from the ouside so i can do something like:
myModule.searchCoords().result
or
myModule.searchCoords()().result
However I still can't access the array (result) from the outside function, let alone from another module.
I've been reading on closures, scopes, nested functions, anonymous functions, etc. but i still can't find the solution. What am i doing wrong?
Problem is, that the query is asynchronous, so it can't return a value in the normal flow. If you pass a function when you call searchCoords, then that function can be called after the results come back - which could be after a long delay. This is necessary to prevent the program flow from being blocked whilst potentially long operations are in process.
// accept a callback function to execute after getting results...
function searchCoords(callback){
var result = result;
connection.query('SELECT * FROM monitoring', function(err, result){
if(err){
console.log(err);
}
// run the callback function, passing the results...
callback({result: result});
});
}
// call like this...
// pass a function accepting results object that will be executed as callback
// once results have been returned...
searchCoords(function(resultsObject){
console.log(resultsObject.result)
})
I've been reading about node.js recently (like many others). I find interesting for some use cases, but am a bit struggling to understand the inner workings, specifically the interaction between closure functions and the process flow of the code.
Let's say I have a function which accepts a key-value array. The function must check that the values follow certain data-quality guidelines (for example some keys must have a value, other keys must have numbers as values etc) before storing the data somewhere (for the purpose of this question let's assume data validation has to be done in the application itself).
In "regular" developments models I'd write something like this:
resultName = validateName(data.name)
resultAddress = validateAddress(data.address)
resultID = validateID(data.id)
if (resultName && resultAddress && resultID) {
store(data)
else {
sendErrorToUser(data)
}
Get the results of the validations, and either explain the error(s) to the user or store data and return some kind of confirmation. The flow is very clear.
The way I understand node.js, the way to do this would be to delegate the validations to a different function (to avoid waiting for each validation to finish), and supply two callback functions to the functions which validate the chunks of data:
* a callback to call when validation is successful
* a callback to call when validation fails
It's easy to now return to the user with a "please wait" message, but I have to wait for all validations to clear (or fail) before storing the data or explaining the problem to the user. As a simple way to figure out if all the validations are done I thought of using a variable that counts the number of functions that called the callback, and emitting a "validation complete" event to store the validated data (or get back to the user with any errors). Or, alternatively, emit an event after each validation is complete and in that event's code check if all validations are complete before emitting the "store" / "error" events.
My question is -- am I approaching this correctly? Or is there a more suitable way to do these kinds of things with node.js (or similar event-based systems).
Thank you!
Alon
Are your validations asynchronous? If they are not you can use the code you posted, the "regular" one.
If the validations are asynchronous (checking uniqueness of an email for instance), you need to provide callbacks:
var validateUniqueEmail = function (data, callback) {
db.find({email: data.email}, function (err, result) {
callback(err, result === null);
})
};
var validateAndStore = function (data, callback) {
asyncValidation(data, function (err, is_valid) {
if (err) {
callback(err, null);
} else if (!is_valid) {
callback('Email is not unique', null);
} else {
db.store(data, callback);
}
});
}
The code above can be simplified a lot by using some validator or ORM modules already existing
example: mongolia validator module.
Let's go. Basically, what you want to do is something along the lines of :
var validate(data, cb){
var allOk = true;
for(var key in data){
allOk = allOk && validate[key](data.key); // validator depends on the key
}
if (allOk) cb(null, data); else cb(new Error "bleh");
}
This could be done the following way (note how we pass the failed keys as the first (error) argument to the callback):
var validate(data, cb){
var status = {true:[], false:[]},
total = Object.keys(data).length,
done = 0;
for (var key in data)
(function(key){
validate[key](data[key], function(ok){
status[ok].push(key);
if (++done == total){
status[false].length ? cb(status[false]) : cb(null);
}
});
})(key);
}
Which you can use this way :
validate(data, function(failures){
if (failures){
// tell the user the input does not validate on the keys stored in failures
} else {
// all ok
store(data);
}
});
Correct me if I'm wrong, but I think what you're asking is how to handle the response from multiple asynchronous calls.
Here's how I do it (using your validation example):
var result = {};
function isAllDataAvailable() {
return result.name !== undefined
&& result.address !== undefined
&& result.id !== undefined;
}
function callback(error) {
if (error) {
showError(error);
// terminate here (?)
return;
}
if (isAllDataAvailable()) {
showOutput();
}
}
validateName(data, callback);
validateAddress(data, callback);
validateEmail(data, callback);
The key here is the result object, which starts out as empty. As each field gets validated, it gets added to the result object (by the validation functions, which I've left out in the above snippet). I've used a single callback method, but you could have multiple, say callbackName, callbackAddress, etc. The validation results are processed only if and when the result object has been fully populated, which is checked in isAllDataAvailable.
Hope this helps.
Consider using: https://github.com/asaf/nodejs-model
It will make your life much easier when dealing with validators.