Async findOne() operation inside forEach Loop

I am having a hard time understanding JavaScript Promises. I am searching on of my Mongoose models for objects that meet a certain condition and if they exist, I want to make the object into a plain JS object and add a property onto it.

Unfortunately, I am unable to wrap my head around how I can ensure my forEach loop will run completely before my promise ends up resolving. Please see my code.

// Called to check whether a user has participated in a given list of challenges
participationSchema.statics.getParticipation = function(user, challenges) {
  return new Promise((resolve, reject) => {
    challengesArray = [];
    challenges.forEach((challenge) => {
      // Model#findOne() is Async--how to ensure all these complete before promise is resolved?
      Participation.findOne({user, challenge})
      .then((res) => {
        if (res) {
          var leanObj = challenge.toObject();
          leanObj.participation = true;
          challengesArray.push(leanObj);
        }
      })
      .catch(e => reject(e));
    })
    console.log("CHALLENGES ARRAY", challengesArray); // Challenges Array empty :(
    resolve(challengesArray);
  });
}

I've looked through similar questions, but am unable to get to an answer. Appreciate the help.

via Govind Rai