A complicated model tree loading function

load-form-tree.js

import Ember from 'ember';
import DS from 'ember-data';

const { RSVP } = Ember;
const { PromiseObject } = DS;

async function loadSections(form) {
  return form.get('formSections');
}

/**
 * Maps over the given resolved FormSection models for their questions.
 * @param  {Array<FormSection>} sections
 * @return {Promise<Array<Question>>} Returns a Promise which resolve to a flattened array of all questions for the given sections.
 */
async function loadQuestions(sections) {
  const questionPromises = sections.map((section) => {
    return section.get('questions');
  });

  return RSVP.all(questionPromises).then((questionManyArrays) => {
    return questionManyArrays.flatten();
  });
}

/**
 * Maps over the given resolved Questions for their sub questions.
 * @param  {Array<Question>} questions
 * @return {Promise} A promise that resolves when all subquestions for the given questions are properly loaded.
 */
async function loadSubQuestions(questions) {
  const subQuestionPromises = questions.map((question) => {
    question.get('subQuestions');
  });

  return RSVP.all(subQuestionPromises);
}

/**
 * Ensures the given model is a PromiseObject so we can call `then` on it.
 * @param  {DS.Model} model Expected to be a subclass of DS.Model.. but maybe it can be any object?
 * @return {PromiseObject}  Returns the model if it's a PromiseObject or creates and returns a PromiseObject from it if not.
 */
function ensurePromiseObject(model) {
  if (model instanceof PromiseObject) {
    return model;
  } else {
    return PromiseObject.create({
      promise: async function() { return model; }
    });
  }
}

/**
 * Load the full model tree for the given form.
 *
 * @param  {Form} formToLoad The form object to load. This may be the loaded form model or a PromiseOjbect, code handles both.
 * @return {Promise<Form>}   Returns a Promise which resolves the given form once all child models are loaded (sections, questions, subQuestions).
 */
export default async function loadFormTree(formToLoad) {
  let formPromise = ensurePromiseObject(formToLoad);
  const form = await formPromise;
  return loadSections(form)
  .then(loadQuestions)
  .then(loadSubQuestions).then(() => {
    return form;
  });
}