I have two collections in mongoose db. I have to find all the documents in a collection. After that, i have to iterate all the documents to find the corresponding document in second collection. For that, i was thinking to use for loop. But, since it is blocking in nature. How can I perform my task.
const docs = await collection1.find({name:"asdf"})
for(let i=0;i<docs.length;i++){
const doc2 = await collection2.findOne({address:docs.address})
}
I'm not sure if I really understand your problem, but if I do, I think you can push all yours promises inside an array, instead of using await. After that you can use the function Promise.all to await in on place for all promises inside the array. You can find below an example with your code:
const docs = await collection1.find({name:"asdf"})
const docs2 = [];
for(let i=0;i<docs.length;i++){
docs2.push(collection2.findOne({address:docs.address}));
}
Promise.all(docs2).then((values) => {
// at this point all your documents are ready
// and your for is not blocking
console.log(values);
});
But you have to be carefull to not abuse of this, if you fill an array with a lot of promises, it can lead to performance/memory issues. Regards
collection2.find() that returns all docs you need instead of individual findOne() calls which are much slower. Basically running findOne() inside any kind of loop, with or without promises, is a database client antipattern and a performance code smell.
await collection2.findOne(...)), the main thread is taking care of "other things". Async/await does not block the event loop, in simple wordsawaitis just sugar-coating the nesting of promises and callbacks so that your code structure looks "flat". Something like reading a file withfs.readFileSync()inside the loop (or anywhere in your code) would block the event loop, but that's not the case withawait.