For my SO question http://stackoverflow.com/q/33175305/4825465

3. Promises way.

// Logic MUST BE identical to the synchronous way:
// https://gist.github.com/melissamarima/2b9e8594892103dbc02c

var mongoose = require('mongoose');
var collectionASchema = new mongoose.Schema({field1: Number, field2: Number});
var collectionBSchema = new mongoose.Schema({field1: Number, field2: Number});
collectionASchema.index({field1: 1});
collectionBSchema.index({field1: -1});
mongoose.model('collectionA', collectionASchema);
mongoose.model('collectionB', collectionBSchema);

collectionBSchema.methods.addToCollectionB = function(cb){
  // `this` is the newDocB
  var newDocB = this;
  var collectionA = mongoose.model("collectionA");

  // set up the recursive promise loop
  // returns nothing -- should break out of this recursion at the end
  function promiseRecursion(){
    // 2. Get the current collection A's first element
    var colAFirstGetPromise = collectionA.find({}).sort({price:1}).limit(1).exec();
    colAFirstGetPromise.then(function (collectionADocsArray){
      var docA = collectionADocsArray[0];

      // 3. check loop condition, break out of loop if satisfied
      if (docA.field2 <= 0){
        return; // EXISTS LOOP! TODO: make sure this breaks out of the loop
      }

      // 4. check which of the two ways to modify newDocB
      if (docA.field1 <= 0){
        // 6. one other way breaks out of the loop
        newDocB.field1 = 0;
        return; // EXISTS LOOP! TODO: make sure this breaks out of the loop
      }
      // else .. 5. one way removes docA from collectionA
      newDocB.field1 -= docA.field1;
      return collectionA.findByIdAndRemove(docA.id).exec();
    }).then(function(doc){
      // doc A should be removed at this point

      // 7. if collectionA is empty, break out of the loop
      return collectionA.find().exec();
    }).then(function(collectionADocsArray){
      if (collectionADocsArray.length == 0){
        return; // EXISTS LOOP! TODO: make sure this breaks out of the loop
      }

      // 8. loads the new docA and continues to the next loop
      return colAFirstGetPromise;
    }).then(function (collectionADocsArray){
      docA = collectionADocsArray[0];
      // loop back
      return promiseRecursion();

    // handles error for all the promises above
    }, function(err){
      return err;
    });
  } // end of promiseRecursion();

  // START HERE!!!!!!!!! THIS IS THE FIRST STEP
  // 1. If collection A is empty, push newDocB onto collection B, and stop
  collectionA.find(function(err, collectionADocsArray){
    if (err){return err;}
    if (collectionADocsArray.length == 0){
      newDocB.save(cb); // saves newDocB to collectionB
      return; // ends addToCollectionB()
    }

    promiseRecursion();

    // 9. Loop is done, check if newDocB needs to be saved
    //TODO: QUESTION QUESTION!!!!!!!!!!
    // Was the newDocB modified in promiseRecursion() the same as the one below???
    if (newDocB.field1 >= 0){
      newDocB.save(cb); // saves newDocB to collectionB
      return; // ends addToCollectionB()
    }
  });
}