Homework 3

Reading

Mongo embedding vs referencing

Let's say for example that you owned a series of bookstores each with a location, a manager, and tons of books. Each book has an author and a price. So our data structure so far looks like

bookstore
  location
  manager
  book
    author
    price

Keep in mind that a lot of books will be repeated across bookstores. How would we convert this to a Mongo datastore? There are 2 main ways: Referencing and Embedding.

Referencing Referencing is when you reference a Mongo document (usually by _id) inside of another document. We could split up the bookstore into two separate Mongo collections, a store and a book. Then our collections will look like

store
  location
  manager
  items

book
  author
  price

This decouples stores with what they carry. We now have 1 book object that can be referenced from multiple stores. This is useful when you are lacking in space (because you don't repeat books). It is also useful when the object being shared changes often. Imagine that this bookstore based the price of their books on the Amazon.com price of the book (which fluctuates constantly). Now every time the price of the book changes you have to make only one change to one object, and the next time a store looks up the book it will see the updated price.

Embedding Embedding is when you store a Mongo document inside of another Mongo document. This is the default way to do things in Mongo, and is the most obvious. Instead of creating two collections for your bookstore, you'll just have one bookstore collection that has a list of every book inside of the bookstore.

bookstore
  location
  manager
  book
    author
    price

This will lead you to repeat books across bookstores (but who cares because space is cheap). However, it will also mean that if you want to change the price of a book across all bookstores you have to go through each bookstore, search for the book, then change the attribute of the book. This isn't too bad if the book changes price very rarely, or if there are only a few stores which stock the book. However, if we think back to the Amazon.com example, if the price of the book changes every hour, and you have 1000 bookstores which stock the book. That means you have to go update 1000 objects every hour, compared to 1 per hour if you had used referencing. This becomes an even bigger problem when you're a product like Twitter and your "bookstores" are users and books are people those users follow. Let's say you want to update information about the book "Lady Gaga", which is stocked by 33 million "bookstores". This would be nearly impossible with embedded data, but is a cynch with references.

In the end which way you use (reference or embedding) depends what your data access patterns will be like. You'll likely be using embeds 80% of the time, but references also have their place, so know how to do both.

The Mongo documentation has further details about when to embed vs reference.

Assignment

Great news! We started shopping around your last two homeworks (hope you don't mind) and although no one was interested in buying your cat tracking software, we did get an email from a local burger join, looking for some help. Jessica's Burgers is looking to update their aging ordering system to the 21st century. So in an effort to make us loads of cash improve your coding skills, this homework will focus on making a web app which will help Jessica's customers get their delicous burgers even quicker (and more delicously). Your application will allow users to build orders for single burger from a list of ingredients (which will need to be updated as new stock arrives). Then it will allow Jessica's chefs to see all the pending orders, fill them, and alert customers that their burger is ready.

Your application will need the following http endpoints:

• /ingredient/new => shows a form that allows someone to add a new ingredient into the system. An ingredient object consists of an ingredient name and a cost, for example {ingredient: cheese, cost:2}.
• /order/new => shows a form which will allow customers to create a new burger. There should be a checklist of all available ingredients and a "submit order" button. Clicking on the "submit order" button should submit the customer's order WITHOUT refreshing the page. i.e., with an AJAX $.post request.
• /orders will show a list of all the pending orders. There should be a "completed" button beside each order that does another $.post request to your server and completes the order. Clicking this button should also remove this order from the list of orders in the browser (again, without refreshing the page).
• Deploy your application to heroku and add your hame to the Homework 3 sheet

Some Help (if you need it)