How to Handle Return Data from Asynchronous MongoDB Queries Within a Loop?

Resolving Return Data from Asynchronous Queries in a Loop

In database applications, it's often necessary to perform multiple queries in a loop and return the aggregated results. When using asynchronous database operations, it becomes crucial to handle the returning data correctly, ensuring that your code waits for all queries to complete before sending a response.

In this scenario, our code aims to query MongoDB for multiple prayer categories and return an array of data containing the category IDs, prayer counts, and other details. However, simply returning inside the loop led to undefined values since the code didn't wait for the asynchronous queries to finish.

To resolve this issue, we can utilize the Q library, which aids in managing asynchronous operations. Unfortunately, the MongoDB library's find method doesn't return a promise, so we employ Q's node-interfacing helpers to create an immediate wrapper around it, ensuring it returns a promise:

var find = Q.nbind(Prayer.find, Prayer);

Following the principles of promise handling, we modify the forEach callback to return promises for each query and then use Q.all to wait for all promises to resolve:

function getPrayerCount(data2) {
    var id = data2.id;
    return find({prayerCat:id})
    .then(function(prayer) {
        if (!prayer)
            data2.prayersCount = 0;
        else
            data2.prayersCount = prayer.length;
        return data2;
    });
}

function getPrayerInCat(data) {
    var promises = data.map(getPrayerCount);
    return Q.all(promises);
}

This approach ensures that our code waits for all queries to complete before returning the aggregated data, preventing undefined values. Remember, when working with asynchronous operations, it's essential to adhere to the rules of promise handling, creating wrappers where needed and utilizing libraries like Q to simplify the process.

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