Julia Evans

Day 2: Rails associations & dragging divs around

Hello! Today was day 2 of building my toy project. Here are a few more notes on things that have been fun about Rails!

the goal: make a refrigerator poetry forum

I wanted to make kind of a boring standard website to learn Rails, and that other people could interact with. Like a forum! But of course if people can actually type on a website that creates all kinds of problems (what if they’re spammers? or just mean?).

The first idea I came up with that would let people interact with the website but not actually be able to type things into it was a refrigerator poetry forum where you can write poems given only a fixed set of words.

So, that’s the plan!

My goal with this project is to find out if I want to use Rails for other small web projects (instead of what I usually do, which is use something more basic like Flask, or give up on having a backend at all and write everything in Javascript).

how do you drag the words around? jQuery UI draggable!

I wanted people to be able to drag the words around, but I didn’t feel like writing a lot of Javascript. It turns out that this is SUPER easy – there’s a jQuery library to do it called “draggable”!

At first the dragging didn’t work on mobile, but there’s a hack to make jQuery UI work on mobile called jQuery UI touch punch. Here’s what it looks like (you can view source if you’re interested in seeing how it works, there’s very little code).

banana forest cake is

a fun Rails feature: “associations”

I’ve never used a relational ORM before, and one thing I was excited about with Rails was to see what using Active Record is like! Today I learned about one of Rails’ ORM features: associations. Here’s what that’s about if you know absolutely nothing about ORMs like me.

In my forum, I have:

  • users
  • topics (I was going to call this “threads” but apparently that’s a reserved word in Rails so they’re called “topics” for now)
  • posts

When displaying a post, I need to show the username of the user who created the post. So I thought I might need to write some code to load the posts and load the user for each post like this: (in Rails, Post.where and User.find will run SQL statements and turn the results into Ruby objects)

@posts = Post.where(topic_id: id)
@posts.each do |post|
    user = User.find(post.user_id)
    post.user = user

This is no good though – it’s doing a separate SQL query for every post! I knew there was a better way, and I found out that it’s called Associations. That link is to the guide from https://guides.rubyonrails.org, which has treated me well so far.

Basically all I needed to do was:

  1. Add a has_many :posts line to the User model
  2. Add a belongs_to :user line to the Post model
  3. Rails now knows how to join these two tables even though I didn’t tell it what columns to join on! I think this is because I named the user_id column in the posts table according to the convention it expects.
  4. Do the exact same thing for User and Topic (a topic also has_many :posts)

And then my code to load every post along with its associated user becomes just one line! Here’s the line:

@posts = @topic.posts.order(created_at: :asc).preload(:user)

More importantly than it being just one line, instead of doing a separate query to get the user for each post, it gets all the users in 1 query. Apparently there are a bunch of different ways to do similar things in Rails (preload, eager load, joins, and includes?). I don’t know what all those are yet but maybe I’ll learn that later.

a fun Rails feature: scaffolding!

Rails has this command line tool called rails and it does a lot of code generation. For example, I wanted to add a Topic model / controller. Instead of having to go figure out where to add all the code, I could just run:

rails generate scaffold Topic title:text

and it generated a bunch of code, so that I already had basic endpoints to create / edit / delete Topics. For example, here’s my topic controller right now, most of which I did not write (I only wrote the highlighted 3 lines). I’ll probably delete a lot of it, but it feels kinda nice to have a starting point where I can expand on the parts I want and delete the parts I don’t want.

database migrations!

The rails tool can also generate database migrations! For example, I decided I wanted to remove the title field from posts.

Here’s what I had to do:

rails generate migration RemoveTitleFromPosts title:string
rails db:migrate

That’s it – just run a couple of command line incantations! I ran a few of these migrations as I changed my mind about what I wanted my database schema to be and it’s been pretty straightforward so far – it feels pretty magical.

It got a tiny bit more interesting when I tried to add a not null constraint to a column where some of the fields in that column were null – the migration failed. But I could just fix the offending records and easily rerun the migration.

that’s all for today!

tomorrow maybe I’ll put it on the internet if I make more progress.

Day 1: a confusing Rails error message Day 3: an infinitely tall fridge