code: function get_association_keys(collection) {
    // Loop over the keys, filter irrelevant ones and fetch alias'
    return Object.keys(collection._attributes || collection.attributes)
      // We only care about attributes with model, collection or foreignKey properties.
      .filter(key =>
        collection._attributes[key].hasOwnProperty("model") ||
        collection._attributes[key].hasOwnProperty("collection") ||
        (
          collection._attributes[key].hasOwnProperty("foreignKey") &&
          collection._attributes[key].hasOwnProperty("references")
        )
      )
  }
docs: Get the keys that are associations.
  @param  {Waterline.Collection} collection to get keys from.
  @return {Array} Array of the keys that have model, collection or foreignKey properties.