Mongo query inside a for-loop nodeJS

Question

I'm trying to make a query that will look for a specific element in an array and print out the object that the specific element resides within. I wanna do this for every element in my dataset

Here's an example of what my dataset looks like (but with around 10k more datasets. These are just the first 2 sets in the database):

/* 1 */
{
    "_id" : ObjectId("5b6b19cb1be7e54a24344bd5"),
    "id" : 18009,
    "ingredients" : [ 
        "baking powder", 
        "eggs", 
        "all-purpose flour", 
        "raisins", 
        "milk", 
        "white sugar"
    ]
}

/* 2 */
{
    "_id" : ObjectId("5b6b19cb1be7e54a24344bd6"),
    "id" : 28583,
    "ingredients" : [ 
        "sugar", 
        "egg yolks", 
        "corn starch", 
        "cream of tartar", 
        "bananas", 
        "vanilla wafers", 
        "milk", 
        "vanilla extract", 
        "toasted pecans", 
        "egg whites", 
        "light rum"
    ]
}

So what I want is, first of all to find the recipes where baking powder exsists, then I wanna print ot those objects where it exists. I wanna do the same for all of the ingredients. I have tried doing following to achieve this:

const path = "mongodb://localhost:27017/NN_Recipes";


mongo.connect(path, function(err,db) {

    console.log("Connected!")


    var allIngredients;


    return new Promise((resolve, reject) => {
        db.collection('recipes_final').distinct("ingredients", function(err, resu) {

            allIngredients = resu;
            resolve();
        })
    }).then(data => {
        for(i = 0; i < allIngredients.length; i++) {

            var currentIngredient = allIngredients[i];

            var individualMatrix = db.collection('recipes_final').find({
                ingredients: currentIngredient
            }).toArray(function(error, response) {
                  // console.log(currentIngredient); //This will only print the very last element in the allIngredients[i], none of the other ones.

            });

            //all below is just things i tried to play around with.
        //     console.log(i)
        //     console.log(individualMatrix)
        //    console.log(allIngredients[i])
        }
       // console.log(allIngredients)
    })


});

anyone who can explain why it's only printing out the last element in my dataset?

you have missed closure inside for loop. like this (function(i) { console.log(i); })(0) — Suresh Mahawar
– Suresh Mahawar, Commented Aug 10, 2018 at 12:05

Jeremy Thille · Accepted Answer · 2018-08-10 12:54:59Z

2

You can try to async/await everything, so every call will be made in order, one after the other :

mongo.connect( path, async (err, db) => {

    const recipes = db.collection('recipes_final'),
           allIngredients = await recipes.distinct("ingredients");

    for (let ingredients of allIngredients) {
        let individualMatrix = await recipes.find({ ingredients }).toArray()
        console.log(individualMatrix)
    }
});

edited Aug 10, 2018 at 12:54

answered Aug 10, 2018 at 11:45

Jeremy Thille

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2 Comments

Thomas Over a year ago

Thank you. This solved my question! Not very used to asyncronous programming, gotta read up on it more. Thanks!

Jeremy Thille Over a year ago

You're very welcome. Async programming requires some time and practice to get used to. I have optimized my code, cached the collection object, etc.

Vipul Solanki · Accepted Answer · 2018-08-10 11:48:02Z

0

Don't use for loop, it isn't waiting for print output. Your for loop is not asynchronous, use async for loop.

async.each(openFiles, saveFile, function(err){
    // if any of the saves produced an error, err would equal that error
});

answered Aug 10, 2018 at 11:48

Vipul Solanki

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Comments

Suresh Mahawar · Accepted Answer · 2018-08-10 12:04:04Z

0

I found out the what is an issue with it, you have missed the closure inside the for loop

for(i = 0; i < allIngredients.length; i++) {
   (function(i){
        var currentIngredient = allIngredients[i];

        var individualMatrix = db.collection('recipes_final').find({
            ingredients: currentIngredient
        }).toArray(function(error, response) {
              // console.log(currentIngredient); //This will only print the very last element in the allIngredients[i], none of the other ones.

        });
       })(i)
    }

answered Aug 10, 2018 at 12:04

Suresh Mahawar

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Comments

UtkarshPramodGupta · Accepted Answer · 2018-08-10 12:24:51Z

0

Use ES6 let or closures.

Database queries in your case is asynchronous. When you are executing a query like:

db.collection('collectionName').find().toArray(callback);

The callback function here is executed at next tick i.e. after the completion of your whole script. So, when the current tick is finished, i (of your for loop) has become equal to allIngredients.length and that is pointing to the last element. You can mitigate this by using a closure or let (if you are using ES6) instead of var to declare your currentIngredient variable as shown below:

for (i = 0; i < allIngredients.length; i++) {
  let currentIngredient = allIngredients[i];
  // rest of your code
}

answered Aug 10, 2018 at 12:24

UtkarshPramodGupta

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