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MongoDB Bulk Insert Operation instead of For-Loop

Let's consider a social network to be built by NodeJS and MongoDB.
So, if a user creates a new post, it should be saved to his/her followers feed.
The straightforward implementation of this operation as follow (simplified):
var newPost="New Post";
//get list of followers of user 1
var listOfFollowers = followersCollection.find({u:"1"});
for(var i=0;i<listOfFollowers.length;i++){
var followerId = listOfFollowers[i]._id;
//insert new post of user 1 to every follower feed
feedsCollection.insertOne(
{ownerId:followerId,authorId:"1",content:newPost}
);
}
This, of course, has very bad performance in case of big numbers in followers count. How can do this with a single fast performing MongoDB query?

MongoDB provides bulk document insert functionality, check out this link - https://docs.mongodb.com/manual/reference/method/db.collection.initializeUnorderedBulkOp/
db.collection.initializeUnorderedBulkOp() It creates an unordered list of operations and mongodb executes this list in parallel, so it's fast and you don't have to take extra care of performance as mongo handles it.
For ordered insertions, you can use db.collection.initializeOrderedBulkOp().
e.g.
var newPost="New Post";
var bulk = db.followersCollection.initializeUnorderedBulkOp();
//get list of followers of user 1
var listOfFollowers = followersCollection.find({u:"1"});
for(var i=0;i<listOfFollowers.length;i++){
var followerId = listOfFollowers[i]._id;
//insert new post of user 1 to every follower feed
bulk.insert( {ownerId:followerId,authorId:"1",content:newPost});
}
bulk.execute();
If you are using Mongoose then checkout Mongoose docs for the same. In the above example, I have just trying to explain how you can do it using plain MongoDB.

Insert Many Read this document I think you will get the answer

Check this:
var newPost="New Post";
//Object Array
var collection = []
//get list of followers of user 1
var listOfFollowers = followersCollection.find({u:"1"});
for(var i=0;i<listOfFollowers.length;i++){
var followerId = listOfFollowers[i]._id;
collection.push({ownerId:followerId,authorId:"1",content:newPost})
}
feedsCollection.insert(collection); //Or use insertMany()
You can create your object array and insert it at once.
Check documentation :- https://docs.mongodb.com/manual/reference/method/db.collection.insert/#insert-multiple-documents
Even though this is a simple answer for your question, If the collection array has a large number of elements, there still might be performance issues. So the best way to handle this is using triggers. https://docs.mongodb.com/stitch/triggers/

Returning a single child's value on Firebase query using orderByChild and equalTo

I am trying to pull a URL for an image in storage that is currently logged in the firebase real time database.
This is for a game of snap - there will be two cards on the screen (left image and right image) and when the two matches the user will click snap.
All of my image urls are stored in the following way:
Each one has a unique child called "index" - I also have another tree that is just a running count of each image record. So currently I am running a function that checks the total of the current count, then performs a random function to generate a random number, then performs a database query on the images tree using orderByChild and an equalTo that contains the random index number.
If I log the datasnap of this I can see a full node for one record (So index, score, url, user and their values) however if I try to just pull the URL I get returned a value of Null. I can, rather annoyingly, return the term "URL" seemingly at my leisure but I can't get the underlying value. I've wondered if this is due to it being a string and not a numeric but I can't find anything to suggest that is a problem.
Please bare in mind I've only been learning Javascript for about a week at max, so if I'm making obvious rookie errors that's probably why!
Below is a code snippet to show you what I mean:
var indRef = firebase.database().ref('index')
var imgRef = firebase.database().ref('images')
var leftImg = document.getElementById('leftImg')
var rightImg = document.getElementById('rightImg')
document.addEventListener('DOMContentLoaded', function(){
indRef.once('value')
.then(function(snapShot){
var indMax = snapShot.val()
return indMax;
})
.then(function(indMax){
var leftInd = Math.floor(Math.random()* indMax + 1)
imgRef.orderByChild('index').equalTo(leftInd).once('value', function(imageSnap){
var image = imageSnap.child('url').val();
leftImg.src=image;
})
})
})

When you execute a query against the Firebase Database, there will potentially be multiple results. So the snapshot contains a list of those results. Even if there is only a single result, the snapshot will contain a list of one result.
Your code needs to cater for that list, by looping over Snapshot.forEach():
imgRef.orderByChild('index').equalTo(leftInd).once('value', function(imageSnap){
imageSnap.forEach(function(child) {
var image = child.child('url').val();
leftImg.src=image;
})
})

meteor.js & mongoDB - query with multiple fields

CONTEXT
I am trying to create a search functionality allowing users to fill in multiple fields, submit, and see a list of matching items from one collection. I do this using a form on the front end, which updates session variables on back-end, which are then passed as query to a mongodb collection.
HOW IT SHOULD WORK
If a user submits a venue size, then venues of that size are shown. If only a location is typed in, then venues within that location are shown. If both a size and a location are submitted, then venues that match both criteria are shown.
HOW IT ACTUALLY WORKS
If nothing is filled in, pressing search yields all items in the collection. Submitting both location and size yields venues that match both criteria. However, filling in only one field and leaving the other empty yields nothing in results. I'm wondering why this might be - it's almost as if the query is searching for a field that literally contains ''... but then why don't I see this behavior when leaving both fields empty? Help much appreciated!
CODE SNIPPET
//Search Form Helper
Template.managevenues.helpers({
venue: function () {
var venueNameVar = Session.get('venueNameVar');
var venueLocationVar = Session.get('venueLocationVar');
if(venueNameVar || venueLocationVar){
console.log(venueNameVar);
console.log(venueLocationVar);
return Venues.find({
venueName: venueNameVar,
'venueAddress.neighbourhood': venueLocationVar
});
} else {
return Venues.find({});
}
});

The answer lies in your query
Venues.find({
venueName: venueNameVar,
'venueAddress.neighbourhood': venueLocationVar
});
If you don't have one of your vars set it will look like this...
{
venueName: undefined,
'venueAddress.neighbourhood':'someVal'
}
So it would match any venue that doesn't have a name and is in some neighborhood.
A better approach would be to only set query criteria if there's a value to search...
var query = {};
if(Session.get('venueNameVar')) {
query.venueName = Session.get('venueNameVar');
}
if(Session.get('venueLocationVar') {
query.venueAddress = {
neighbourhood : Session.get('venueLocationVar');
}
}
return Venues.find(query);
I think this will work a bit better for you!

adding list to array of list in localstorage

I have list of students like as follows form from user input form:
//student form input for 1st time
var student={name:"a",roll:"9",age:13}
//student form input for 2nd time
var student={name:"b",roll:"10",age:14}
//student form input for 3rd time
var student={name:"c",roll:"11",age:15}
Actually, i am developing phonegap applications. Each time the user submit form-input i.e. student information, I want to save them into localstorage. Finally, when online, i want to sync them. I know i can store them in localstorage as follows:
localStorage.setItem("studentinfo", JSON.Stringfy(student));
But, this will remove the first student info in the local storage when i save second student info.
Infact, when i save first, second and third input respectively, i want to add them in localstorage array and finally the result in localstorage should be like
key=studentlist,
value=[
{name:"a",roll:"9",age:13},
{name:"b",roll:"10",age:14},
{name:"c",roll:"11",age:15}
]
How can it be done in localstorage or phonegap localstorage?

You want to hold all your students in an array like this:
var students = [];
students.push({name:"a",roll:"9",age:13});
students.push({name:"b",roll:"10",age:14});
students.push({name:"c",roll:"11",age:15});
And then store that in localStorage:
localStorage.setItem('studentsInfo', JSON.stringify(students));
The best way to do that would be with a function like this:
// When you get more student information, you should:
var addNewStudent = function (name, roll, age) {
// retrieve it (Or create a blank array if there isn't any info saved yet),
var students = JSON.parse(localStorage.getItem('studentsInfo')) || [];
// add to it,
students.push({name: name, roll: roll, age: age});
// then put it back.
localStorage.setItem('studentsInfo', JSON.stringify(students));
}

Fetch all Parse.Relations pointing to a given object

I'm using a Parse.Relation to group users under a Topic. How do I retrieve all the topics which have a topic.relation pointing to a given user?
The question is about doing this in a single call/callback.
// first add() a User object to the Topic's Parse.Relation
this.friendRelation = this.topic.relation("friend");
this.friendRelation.add(user);
// save the Topic to save its newly added .relation to Parse/Mongo
this.topic.save();
// iterate by adding the same User to several Topics
// (...)
// then you want to retrieve all Parse.Relations of all Topics where that
// specific user is pointed to
// the non-optimized way is to traverse every row in the Topics index
// and query each topic for its relations to find matching pointers to our user,
// which means that the number of calls is bound to the number of rows – fine for
// 10 topics in Mongo but passed 100 it won't be tolerable.

Construct a query for the Topic class, and add a equalTo constraint.
var query = new Parse.Query(Topic);
query.equalTo("friend", user);
query.find({success: function (returnedTopics) {
...
},
error: function (...) {
}
});
This will return all Topic objects that contain user in their friend relation.