CODEWITHSUNDEEP
node.jsamazon-web-servicesaws-lambdaamazon-dynamodb
James Campbell
James Campbell

Reputation: 83

dynamoDB DocumentClient not waiting inside for loop - node js 8.10

I'm having problems with dynamoDB not giving me the result of a query within a for loop. The query does execute, but it only executes after the loop has completed:

readMatchData(JSON) {
return new Promise((resolve, reject) => {



    for (var jsonInfo of JSON.Feed.MatchData) {


        var matchID = jsonInfo['@attributes'].matchID;

        console.log("matchID: " + matchID);

        var homeTeamID = team['@attributes'].homeTeamID;


        var params = {
            TableName: "Teams",
            KeyConditionExpression: "#teamID = :teamID",
            ExpressionAttributeNames: {
                "#teamID": "teamID"
            },
            ExpressionAttributeValues: {
                ":teamID": homeTeamID
            },
            ReturnConsumedCapacity: "TOTAL"
        }

        docClient.query(params, function(err, data) {

            if (err) {
                //console.log("We have an error when looking for the team in the Teams table");
                console.log(err);

            } else {

                if (data.Count === 0) {

                    //We did not find this ID in the db so we can add it.

                    console.log("The team doesn't exist");

                } else {

                    data.Items.forEach(function(item) {

                        console.log("Team " + item.teamID + " " + item.teamName + " found in the database");

                    })

                }

            }

        });


    }




    resolve("done");

});
}

In the console this returns me:

matchID: 3434

matchID: 3543

Team 3388 Hill U23 Team found in the database

Team 44108 Bridge U23 found in the database

Rather than:

matchID: 3434

Team 3388 Hill U23 Team found in the database

matchID: 3543

Team 44108 Bridge U23 found in the database

Upvotes: 2

Views: 1783

Answers (1)

OllysCoding
OllysCoding

Reputation: 312

The issue is that the docClient call uses a callback as seen in this line:

docClient.query(params, function(err, data) { ... })

This means that the function you're passing is not called until the operation is complete, which can be some time later.

You can tell the library to wrap it in a promise and then await it:

const data = await docClient.query(params).promise()

if (data.Count === 0) {
  //We did not find this ID in the db so we can add it.
  console.log('The team doesn\'t exist')
} else {
  data.Items.forEach(function (item) {
    console.log('Team ' + item.teamID + ' ' + item.teamName + ' found in the database')
  })
}

Edit: Currently you're wrapping your whole function in a promise, but since we're isolating that to just the callback, you probably want to replace that to look something more like:

async function readMatchData () {
  for (var jsonInfo of JSON.Feed.MatchData) {

    var matchID = jsonInfo['@attributes'].matchID

    console.log('matchID: ' + matchID)

    var homeTeamID = team['@attributes'].homeTeamID

    var params = {
      TableName: 'Teams',
      KeyConditionExpression: '#teamID = :teamID',
      ExpressionAttributeNames: {
        '#teamID': 'teamID'
      },
      ExpressionAttributeValues: {
        ':teamID': homeTeamID
      },
      ReturnConsumedCapacity: 'TOTAL'
    }

    const data = await docClient.query(params).promise()

    if (data.Count === 0) {
      //We did not find this ID in the db so we can add it.
      console.log('The team doesn\'t exist')
    } else {
      data.Items.forEach(function (item) {
        console.log('Team ' + item.teamID + ' ' + item.teamName + ' found in the database')
      })
    }
  }
  return 'done'
}

EDIT: Credit to HMilbradt for pointing out the library has a built in promisify function

Upvotes: 4

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