In MongoDB using Express, how can I ensure that a code is only executed after all queries inside a loop have been completed?

Question

In MongoDB using Express, how can I ensure that a code is only executed after all queries inside a loop have been completed?

Here's a snippet of code from my router:

router.get('/questions/best', function(req, res, next) {
    Question.aggregate([
        // Include data protection measures with $ifNull and max[$size,1]       
        {$project: {ratio: {$divide:[
            {$arrayElemAt:[{$ifNull:["$likes",[0]]}, 0]},
            {$max:[1, {$arrayElemAt:[{$ifNull:["$dislikes",[0]]}, 0]}]}
        ]}},
        {$sort: {ratio: -1}},
        {$limit: 15}
    ]).exec(function(err, results) {
        if (err) {return next(err)}
        
        let top15 = [];
        for(let i = 0; i < results.length; i++) {
            Question.findOne({_id: results[i]._id}).exec(function(err, result) {
                if (err) {return next(err)}
                top15.push(result);
                
                if (top15.length === results.length) {
                    res.render('content', { whichOne: 5, user: req.user, questions: top15 });
                }
            })
        }
    });
});

I'm facing an issue where the template gets rendered automatically before all queries are completed, causing incomplete data to be displayed. I've tried using setTimeout but it's not reliable due to varying processing times. Is there a way to ensure that the template is rendered only after all queries have returned and their values are populated in the array? Any suggestions or help would be greatly appreciated. Thank you!

javascript mongodb express mongoose promise

Answer 1

Answer №1

After going through the mongoose documentation, it became clear that using promises for handling asynchronous operations is quite straightforward.

Implementing Promises:

router.get('/questions/best', (req,res,next) => {
    Question.aggregate([
        // Handle potential missing fields with $ifNull                                                         
        // Prevent division by zero dislikes using max[1,size]                                         
        {$project: {ratio: {$divide:[
        {$arrayElemAt:[{$ifNull:["$likes",[0]]}, 0]},
            {$max:[1, {$arrayElemAt:[{$ifNull:["$dislikes",[0]]}, 0]}]}
            ]}
        }},
        {$sort: {ratio:-1}},
        {$limit: 15}
    ]).exec()
    .then(results => Promise.all(results.map(({_id}) => Question.findOne({_id}).exec())))
    .then(questions => res.render('content', { whichOne: 5, user: req.user, questions}))
    .catch(next);
});

Alternatively, utilizing async/await:

router.get('/questions/best', async (req,res,next) => {
    try {
        const results = await Question.aggregate([
            // Handle potential missing fields with $ifNull                                                         
            // Prevent division by zero dislikes using max[1,size]                                         
            {$project: {ratio: {$divide:[
            {$arrayElemAt:[{$ifNull:["$likes",[0]]}, 0]},
                {$max:[1, {$arrayElemAt:[{$ifNull:["$dislikes",[0]]}, 0]}]}
                ]}
            }},
            {$sort: {ratio:-1}},
            {$limit: 15}
        ]).exec();
        const questions = await Promise.all(results.map(({_id}) => Question.findOne({_id}).exec()));
        res.render('content', { whichOne: 5, user: req.user, questions});
    } catch(err) {
        next(err);
    }
});

Answer 2

After going through the mongoose documentation, it became clear that using promises for handling asynchronous operations is quite straightforward.

Implementing Promises:

router.get('/questions/best', (req,res,next) => {
    Question.aggregate([
        // Handle potential missing fields with $ifNull                                                         
        // Prevent division by zero dislikes using max[1,size]                                         
        {$project: {ratio: {$divide:[
        {$arrayElemAt:[{$ifNull:["$likes",[0]]}, 0]},
            {$max:[1, {$arrayElemAt:[{$ifNull:["$dislikes",[0]]}, 0]}]}
            ]}
        }},
        {$sort: {ratio:-1}},
        {$limit: 15}
    ]).exec()
    .then(results => Promise.all(results.map(({_id}) => Question.findOne({_id}).exec())))
    .then(questions => res.render('content', { whichOne: 5, user: req.user, questions}))
    .catch(next);
});

Alternatively, utilizing async/await:

router.get('/questions/best', async (req,res,next) => {
    try {
        const results = await Question.aggregate([
            // Handle potential missing fields with $ifNull                                                         
            // Prevent division by zero dislikes using max[1,size]                                         
            {$project: {ratio: {$divide:[
            {$arrayElemAt:[{$ifNull:["$likes",[0]]}, 0]},
                {$max:[1, {$arrayElemAt:[{$ifNull:["$dislikes",[0]]}, 0]}]}
                ]}
            }},
            {$sort: {ratio:-1}},
            {$limit: 15}
        ]).exec();
        const questions = await Promise.all(results.map(({_id}) => Question.findOne({_id}).exec()));
        res.render('content', { whichOne: 5, user: req.user, questions});
    } catch(err) {
        next(err);
    }
});

In MongoDB using Express, how can I ensure that a code is only executed after all queries inside a loop have been completed?

Answer №1

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