code: function get_related_columns(collection) {
    return Object.keys(collection.attributes)
      .filter(attribute => collection.attributes[attribute].collection || collection.attributes[attribute].model)
  }
docs: Get related column names from a collection.
  Does not include foreign key declarations.
  
  @param  {Waterline.Collection} collection to get related attributes of.
  @return {Array<string>} array of related attribute names.