Unlike findOneAndUpdate, this returns both the old and the new docs, the operation is performed atomically

mongoose-upsert-plugin.js

/*
  Unlike findOneAndUpdate, this returns both the old and the new docs,
  the operation is performed atomically.

  Note that although it can apply defaults properly, it does not apply
  pre-update hooks, with the exception of timestamps (see note in code).

  Example:

    Dogs.upsert({owner: 'wallace', name: 'grommit'}, {breakfast: 'eggs'})
      .then(res => console.dir({'old': res.oldDoc, 'new': res.newDoc}));

*/

module.exports = function UpsertPlugin(schema){

  schema.statics.upsert = function(query, update){
    update = update.$set || update;
    
    let preemptiveData = (new this(query)).toJSON();
    Object.keys(update).forEach(k => delete preemptiveData[k]); // need fields to be unique for $set/$setOnInsert
    
    return this.findOneAndUpdate(query, {
      $set: update,
      $setOnInsert: preemptiveData
    }, {
      upsert: true,
      new: false,
      setDefaultsOnInsert: false // we have explicitly dealt with this above 
    }).then(oldDoc => {
      // Deal with timestamps on the collection - this is rather hacky and 
      // actually means the values stored will be ever so slightly earlier 
      // than the values returned to the caller of this function.
      let timestamps = schema.options.timestamps;
      if(timestamps && timestamps.updatedAt){
        update[timestamps.updatedAt] = Date.now();
      }
      if(!oldDoc && timestamps && timestamps.createdAt){
        update[timestamps.createdAt] = Date.now();
      }

      let newDoc = new this(Object.assign(oldDoc ? oldDoc.toJSON() : preemptiveData, update));
      newDoc.isNew = false; // we have upserted it!!!
      
      return {
        oldDoc,
        newDoc: newDoc
      };
    });
  
  };

};