Nested Async/Await Loop

user8war 0
Viewed 31

Here is a snippet of my code:

for( const phase of user.phases) {
    for (const schedule of phase.schedules) {
      const populatedSchedule = await db.collection('calendar').find({_id: schedule.calendar_id});
      console.log('populatedSchedule')
      console.log(populatedSchedule.toArray()) // THIS IS A PENDING PROMISE...WHY??
    }
  }

Note: the code above is in an async function.

I have a nested for-of. I expected the nested loop to "await" the result of db.collection.find. However, an unresolved promise is returned.

I thought for-of loops executed sequentially and would wait for the previous loop to finish, however I think the outer loop isn't allowing the nested loop to resolve the await.

async-await mongodb
1 Answers

Where to start....

1. Javascript mongodb native driver

Collection.find returns a cursor, not a document. https://mongodb.github.io/node-mongodb-native/4.9/classes/Collection.html#find It's sync operation, so await here is useless and has no effect. To retrieve the documents you need either iterate the cursor (this is async, and this is where you need to await) or get all documents from the cursor with toArray() https://mongodb.github.io/node-mongodb-native/4.9/classes/FindCursor.html#toArray, it returns a promise and you need to await it.

Since you are querying by _id, there is no more than 1 matching document, so you can use findOne() https://mongodb.github.io/node-mongodb-native/4.9/classes/Collection.html#findOne - it hides the cursor within and returns the promise straight away:

const populatedSchedule = await db.collection('calendar').findOne({_id: schedule.calendar_id});

2. Javascript async/await

Using await within a loop is extremely counter productive. You query 1 document at a time, wait for response, then query the next. You loose all benefits of the connection pool and parallel requests. Please learn Promise.all https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise/all it let you start number of request simultaneously and resolve the promise when all queries are resolved. Then you can iterate the results.

const schedules = await Promise.all(phase.schedules.map(
    schedule => db.collection('calendar').findOne({_id: schedule.calendar_id});
))
for (const schedule of schedules) {
  console.log('populatedSchedule');
  console.log(schedule)
}

3. Mongodb queries

1 find query to match multiple ids is way more efficient than multiple findOne queries. Learn $in operator https://www.mongodb.com/docs/manual/reference/operator/query/in/

const ids = phase.schedules.map( schedule => schedule.calendar_id);
const populatedSchedules = await db.collection('calendar').find({_id: {$in: ids}}).toArray();
for (const schedule of schedules) {
  console.log('populatedSchedule');
  console.log(schedule)
}

The Promise.all rule applies here too.

Instead of 2 nested loops with await inside, it should be 1 await for all promises outside:

const results = await Promise.all(user.phases.map( phase => {
    const ids = phase.schedules.map( schedule => schedule.calendar_id);
    return db.collection('calendar').find({_id: {$in: ids}}).toArray();  // <== NO AWAIT HERE IS IMPORTANT!
})
Alex Blex0
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