Add migration for unique username attribute

migrations/20170211214917-unnamed-migration.js

@@ -0,0 +1,17 @@
+'use strict';
+
+module.exports = {
+  up: function (queryInterface, Sequelize) {
+    return queryInterface.changeColumn('users', 'username', {
+      type: Sequelize.STRING,
+      unique: true
+    })
+  },
+
+  down: function (queryInterface, Sequelize) {
+    sequelize.query(
+      'ALTER TABLE users DROP CONSTRAINT username_unique_idx;'
+    );
+    queryInterface.removeIndex('Users', 'username_unique_idx');
+  }
+};