add index.js for model management

backend/src/models/index.js

@@ -0,0 +1,29 @@
+const { sequelize } = require('../config/db');
+const User = require('./User');
+const Whiskey = require('./Whiskey');
+const Collection = require('./Collection');
+const Rating = require('./Rating');
+
+// Define relationships
+
+User.hasMany(Collection);
+Collection.belongsTo(User);
+
+Whiskey.hasMany(Collection);
+Collection.belongsTo(Whiskey);
+
+User.hasMany(Rating);
+Rating.belongsTo(User);
+
+Whiskey.hasMany(Rating);
+Rating.belongsTo(Whiskey);
+
+// Exports
+
+module.exports = {
+  User,
+  Whiskey,
+  Collection,
+  Rating,
+  sequelize
+};