ES6 refactor: associations (#6050)

* Make BelongsTo association an ES6 class

* ES6 refactor of belongs-to.js

* Make BelongsToMany an ES6 class

* ES6 refactor of BelongsToMany

* Make HasMany an ES6 class

* ES6 refactor of HasMany

* Make HasOne an ES6 class

* ES6 refactor of HasOne

* ES6 refactor of association helpers

* ES6 refactor of associations/index.js

* ES6 refactor of association mixin

lib/associations/helpers.js

 'use strict';
 
-var Utils = require('./../utils');
+const Utils = require('./../utils');
 
 function checkNamingCollision (association) {
   if (association.source.rawAttributes.hasOwnProperty(association.as)) {
@@ -11,6 +11,7 @@ function checkNamingCollision (association) {
     );
   }
 }
+exports.checkNamingCollision = checkNamingCollision;
 
 function addForeignKeyConstraints (newAttribute, source, target, options, key) {
   // FK constraints are opt-in: users must either set `foreignKeyConstraints`
@@ -19,9 +20,9 @@ function addForeignKeyConstraints (newAttribute, source, target, options, key) {
   if (options.foreignKeyConstraint || options.onDelete || options.onUpdate) {
 
     // Find primary keys: composite keys not supported with this approach
-    var primaryKeys = Utils._.chain(source.rawAttributes).keys()
-      .filter(function($key) { return source.rawAttributes[$key].primaryKey; })
-      .map(function($key) { return source.rawAttributes[$key].field || $key; }).value();
+    const primaryKeys = Utils._.chain(source.rawAttributes).keys()
+      .filter($key => source.rawAttributes[$key].primaryKey)
+      .map($key => source.rawAttributes[$key].field || $key).value();
 
     if (primaryKeys.length === 1) {
       if (!!source.$schema) {
@@ -42,8 +43,4 @@ function addForeignKeyConstraints (newAttribute, source, target, options, key) {
     }
   }
 }
-
-module.exports = {
-  checkNamingCollision: checkNamingCollision,
-  addForeignKeyConstraints: addForeignKeyConstraints
-};
+exports.addForeignKeyConstraints = addForeignKeyConstraints;

lib/associations/index.js

 'use strict';
 
-var Association = require('./base');
+const Association = require('./base');
 Association.BelongsTo = require('./belongs-to');
 Association.HasOne = require('./has-one');
 Association.HasMany = require('./has-many');
 Association.BelongsToMany = require('./belongs-to-many');
 
 module.exports = Association;
+module.exports.default = Association;
+module.exports.Association = Association;