如何使用云函数对 Firebase 实时数据库、Node.js 和 Javascript 进行排序和过滤以获得“高分”排行榜

Question

I've been researching and doing trial & error for days but haven't been able to get anything working. In testing, I created a few test nodes in my Firebase Realtime database in order to try to work out the function's logic. My file tree looks like this:

In simply trying to get the basics working, I created a few users under the TestUsers node that each have a very simple child structure of Statistics and then an orbs value.

What I'm trying to do:

Upon updating the orbs value under any of the users, I want to write (well, technically 'update') the Output/firstPlace children to include the username and the number of orbs for the top 3 users (that have the highest orb count out of all the users under TestUsers .

Seems super simple, and trust me, I know there's tons of documentation out there on sorting and filtering Firebase Realtime Database data but I haven't been able to piece this together from days of reading this documentation.

I'm using VS Code to write my functions and am deploying them to my Firebase project.

The function I've written to try to do the above is this:

//Define a new function with a base path and set it to run when a child of this path is changed
exports.testTopPlayers1 = functions.database.ref('/TestUsers/{user}/Statistics').onUpdate(_ => { 

    var allPlayersRef = admin.database().ref('/TestUsers/{user}'); //When this function runs, assign a new path of all users

        //Get a snapshot of the top 3 users with the highest orb count
        allPlayersRef.orderByChild('orbs').limitToLast(3).once('value', (snap) => { 

          //For each child from the above snap (should be 3)...
          snap.forEach((child, context) => {

                const username = context.params.user; //Assign the username to a const
                const afterOrbAmount = child.after.val();
                const orbAmount = afterOrbAmount.orbs; //Assign the number of orbs to a const

                //Return (Promise?) by updating the data in '/Output/firstPlace' with the username and orb amount.
                return admin.database().ref('/Output/firstPlace').update({[username]: [orbAmount]});
          });
  })
})

I would expect that with .forEach , it should run the writing to '/Output/firstPlace portion 3 times, effectively giving me 3 new key-value pairs. But nothing happens.

Based on what I'm trying to do, can you either correct my code or show me the proper way to achieve sorting high scores like this on the backend?

Answer 1

You're not returning anything from the top level of your code, which means Cloud Functions has no way to know when your code is done. Hence: it stop your code after the last } , which is before the database read has been completed.

Your code should look something like this:

exports.testTopPlayers1 = functions.database.ref('/TestUsers/{user}/Statistics').onUpdate(_ => { 

    var allPlayersRef = admin.database().ref('/TestUsers/{user}'); //When this function runs, assign a new path of all users

    let query = allPlayersRef.orderByChild('orbs').limitToLast(3)
    return query.once('value').then((snap) => { 
        let promises = [];
        snap.forEach((child, context) => {
            const username = context.params.user;
            const afterOrbAmount = child.after.val();
            const orbAmount = afterOrbAmount.orbs;

            promises.push(admin.database().ref('/Output/firstPlace').update({[username]: [orbAmount]}));

        });
        return Promise.all(promises);
    })
})

There may be more problems in your code, but this ensures that your Cloud Function will keep running until the query and the subsequent updates have completed.

I highly recommend studying the follow to learn more about how to control when a Cloud Function terminates:

• The documentation on synchronous code, asynchronous code, and promises .
• Doug's great video series on JavaScript promises in Cloud Functions .