gistfile1.txt

var db = require('../models')
var Promise = require('bluebird')

req.User.getPosts({
    include: [
      { model: db.User, attributes: ['displayName', 'name', 'profilePicture'] },
      { model: db.Game, attributes: ['name'] }
    ]
})

.then(function(posts) {

  var promises = []
  var post
  
  posts.forEach(function(p) {     // JS Closure

    promises.push(
      Promise.all([               // Declare and resolve 2nd array of promises for each extra data attribute being fetched from DB, then .spread() it for each result
        db.PostLikes.count({ where: { PostId: p.id } }),
        db.Comment.count({ where: { PostId: p.id } })
      ])
      .spread(function(likes, comments) {   // Maps results from the 2nd promise array
        post = p.toJSON()
        post.likes = likes
        post.comments = comments
        return post
      })
    )
  })
  return Promise.all(promises) // Resolve 1st promises array

}).then(function(posts) {
  res.send(posts)
})

This was hugely helpful in understanding promises, thanks for taking the time to post and update!