Using async.each with promises - async each never stops

Question

I am building this function in node that is a mix of promises querying a DB and a async each. The issue is that the async.each function never stops. The promise2 function is never triggered. Is it an issue in my code or cant an async.each function be mixed with promises? Is there a better way to code what I want to do?

Thanks a lot for your help

promise1().then(function(arr1){
  async.each(arr1, function(obj1, callback) {
    build_select_request(obj1).then(function(select_request){
      query_database(select_request).then(function(result){
        if (result){                       
          build_update_request(obj1).then(function(update_request){                        
            do_query(update_request).then(function(result){                                          
              callback(null)
            }, function(error){
              callback(error)
            })
          }, function(error){
            callback(error)
          })
        } else {
          build_insert_request(obj1).then(function(insert_request){            
            do_query(insert_request).then(function(result){                                                                      
              callback(null)
            }, function(error){
              callback(error)
            })
          }, function(error){
            callback(error)
          })
        }
      }, function(error){
        callback(error)
      })
    }, function(error){
      callback(error)
    })
  }, function(err) {
    // if any of the file processing produced an error, err would equal that error
    if (err) {
      // One of the iterations produced an error.
      // All processing will now stop.
      reject(err)
    } else {
      promise2().then(function(success){        
        resolve(success)
      }, function(error){        
        reject(error)
      })
    }
  })
}, function(error){  
  reject(error)
})

you don't need async if you are using promises, look at bluebird.js http://bluebirdjs.com/docs/api/promise.each.html — Dayan Moreno Leon, Oct 21 '16 at 04:49
@JLRishe, could you help me to rewrite this code in a nicer way? I don't understand the example given on bluebird.js I don't think promise.each is relevant for my case. — Quentin Del, Oct 21 '16 at 05:59

score 3 · Answer 1 · edited May 23 '17 at 12:33

Is it an issue in my code or cant an async.each function be mixed with promises?

It can, but it should not. Writing promise code and then falling back to callbacks results in, well, what you currently have.

Instead, make sure to use proper promise chaining:

var promise = build_select_request(obj1).then(function(select_request){
  return query_database(select_request).then(function(result){
    if (result){                       
      return build_update_request(obj1).then(do_query);
      /* .then(function(update_request){                        
        return do_query(update_request);
      }) */
    } else {
      return build_insert_request(obj1).then(do_query);
      /* .then(function(insert_request){            
        do_query(insert_request)
      }) */
    }
  })
})

You now can use that with async.each like this:

async.each(arr1, function(obj1, callback) {
  var promise = …;
  promise.then(function(result) {      
    callback(null, result)
  }, function(error) {
    callback(error);
  });
}, function(err) {
  …
})

but you better avoid this and instead use Promise.all which also lets you avoid the Promise constructor antipattern that you fell for (given the resolve/reject calls in your outermost callbacks):

promise1().then(function(arr1) {
  return Promise.all(arr1.map(function(obj1) {
    var promise = …;
    return promise;
  }));
}).then(function(results) {
  …
}, function(err) {
  …
});

score 0 · Answer 2 · answered Oct 21 '16 at 05:50

I was missing "return"

promise1().then(function(arr1){
  async.each(arr1, function(obj1, callback) {
    build_select_request(obj1).then(function(select_request){
      return query_database(select_request).then(function(result){
        if (result){                       
          return build_update_request(obj1).then(function(update_request){                        
            return do_query(update_request).then(function(result){                                          
              callback(null)
            }, function(error){
              callback(error)
            })
          }, function(error){
            callback(error)
          })
        } else {
          return build_insert_request(obj1).then(function(insert_request){            
            return do_query(insert_request).then(function(result){                                                                      
              callback(null)
            }, function(error){
              callback(error)
            })
          }, function(error){
            callback(error)
          })
        }
      }, function(error){
        callback(error)
      })
    }, function(error){
      callback(error)
    })
  }, function(err) {
    // if any of the file processing produced an error, err would equal that error
    if (err) {
      // One of the iterations produced an error.
      // All processing will now stop.
      reject(err)
    } else {
      promise2().then(function(success){        
        resolve(success)
      }, function(error){        
        reject(error)
      })
    }
  })
}, function(error){  
  reject(error)
})

Using async.each with promises - async each never stops

2 Answers2