Nodejs mongdb query data using async/await

Question

I'm using node module mongodb v3.2.7, and using express to implement my api

I use callback. And, it works well

The function findItem

const findItem = function (id, callback) {
  if (db) {
    const collection = db.collection('transaction')
    collection.find({ user_id: id})
      .project({ ready: 1, online: 1, status: 1})
      .toArray(function (err, docs) {
        if (err) {
          callback(err)
        } else {
          callback(null, docs)
        }
      })
  }
}

My api

app.get('/', (req, res) => {
  try {
    if (req.query.user_id) {
      const contentType = 'application/json'

      res.set({ 'Access-Control-Allow-Origin': '*',
        'Content-Type': contentType })

      const user_id = parseInt(req.query.user_id, 0)
      findItem(user_id , function (err, docs) {
        if (err) {
          res.status(500).end()
          return res.send(err)
        }
        const data = processData(docs)
        console.log('data', data)
        return res.send(data)
      })
    } else {
      res.send()
    }
  } catch (ex) {
    console.log(ex)
  }
})

Then, I tried to use async/await instead of callback. But, it doesn't work. What mistake I did?

Function findItemAsyncAwait

const findItemAsyncAwait = async function (id) {
  if (db) {
    const collection = db.collection('transaction')
    var docs = await collection.find({ user_id: id })
      .project({ ready: 1, online: 1, status: 1})
      .toArray()
    return docs
  }
}

My changed api

app.get('/', (req, res) => {
  try {
    if (req.query.user_id) {
      const contentType = 'application/json'

      res.set({ 'Access-Control-Allow-Origin': '*',
        'Content-Type': contentType })

      const user_id = parseInt(req.query.user_id, 0)
      const promiseDocs = findItemAsyncAwait(user_id)
      promiseDocs.then((docs) => {
        console.log('docs', docs) // <= docs [], docs is an empty array without any document
        const data = processData(docs)
        console.log('data', data)
        return res.send(data)
      }).catch((err) => {
        res.status(500).end()
        return res.send(err)
      })
    } else {
      res.send()
    }
  } catch (ex) {
    console.log(ex)
  }
})

I still got stuck on this problem. Could anyone help?

Andrea Franchini · Accepted Answer · 2019-07-31 10:53:27Z

1

You have to wait the collection

const findItemAsyncAwait = async function (id) {
  if (db) {
    return await db.collection('transaction')
      .find({ user_id: id })
      .project({ ready: 1, online: 1, status: 1})
      .toArray()
  }
}

answered Jul 31, 2019 at 10:53

Andrea Franchini

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5 Comments

Vu Le Anh Over a year ago

This not only doesn't work, but also raise eslint warning Redundant use of await on a return value. The docs still is an empty array

Andrea Franchini Over a year ago

The eslint warning is correct most of the times - it should be a direct return db.collection() and the function can be declared without async - more info here. For what concern the functionality, db.collection() used without callback should behave like a Promise.

Vu Le Anh Over a year ago

With callback approach, it works well. So, I think there is no problem at line const collection = db.collection('transaction') and collection.find({ user_id: id})...

Andrea Franchini Over a year ago

Try with console.log(db.collection('transaction')) - if it's a Promise you have to do const collection = await db.collection('transaction')

Vu Le Anh Over a year ago

it's an object, not a Promise.

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