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

2. Async function.js

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");

  // 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()
    }

    // 2. Get the current collection A's first element
    collectionA.find({}).sort({price:1}).limit(1).exec(function (err, collectionADocsArray){
      if (err) {return err;}
      var docA = collectionADocsArray[0];

      // faking a while in async http://stackoverflow.com/a/15902899/4825465
      (function loop(){
        // 3. check loop condition
        if (docA.field2 > 0){

          // 4. check which of the two ways to modify newDocB
          if (docA.field1 > 0){
            // 5. one way removes docA from collectionA
            newDocB.field1 -= docA.field1;
            collectionA.findByIdAndRemove(docA.id, function(err, doc){
              if (err) {return err;}
              // docA is removed
              // I think async breaks the continuity since this point
            });
          } else {
            // 6. the other way breaks out of the loop
            newDocB.field1 = 0;
            return; // I hope this return breaks out of loop
          }

          // 7. if collectionA is empty, break out of the loop
          collectionA.find(function(err, collectionADocsArray){
            if (err){return err;}
            if (collectionADocsArray.length == 0){
              return; // I hope this return breaks out of loop
            }
          });

          // 8. loads the new docA and continues to the next loop
          collectionA.find({}).sort({price:1}).limit(1).exec(function (err, collectionADocsArray){
            if (err) {return err;}
            docA = collectionADocsArray[0];
            loop();
          });
        }
      }());

      // 9. Loop is done, check if newDocB needs to be saved
      if (newDocB.field1 >= 0){
        newDocB.save(cb); // saves newDocB to collectionB
        return; // ends addToCollectionB()
      }
    });
  });
}