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+[#]: collector: (lujun9972)
+[#]: translator: ( )
+[#]: reviewer: ( )
+[#]: publisher: ( )
+[#]: url: ( )
+[#]: subject: (Day 2: Rails associations & dragging divs around)
+[#]: via: (https://jvns.ca/blog/2020/11/10/day-2--rails-associations---dragging-divs-around/)
+[#]: author: (Julia Evans https://jvns.ca/)
+
+Day 2: Rails associations & dragging divs around
+======
+null
+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][1]. 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][2]. 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
+end
+```
+
+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][2]. 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][3] 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][4], 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.
+
+--------------------------------------------------------------------------------
+
+via: https://jvns.ca/blog/2020/11/10/day-2--rails-associations---dragging-divs-around/
+
+作者：[Julia Evans][a]
+选题：[lujun9972][b]
+译者：[译者ID](https://github.com/译者ID)
+校对：[校对者ID](https://github.com/校对者ID)
+
+本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出
+
+[a]: https://jvns.ca/
+[b]: https://github.com/lujun9972
+[1]: https://github.com/furf/jquery-ui-touch-punch
+[2]: https://guides.rubyonrails.org/association_basics.html
+[3]: https://blog.bigbinary.com/2013/07/01/preload-vs-eager-load-vs-joins-vs-includes.html
+[4]: https://github.com/jvns/refrigerator-forum/blob/776b3227cfd7004cb1efb00ec7e3f82a511cbdc4/app/controllers/topics_controller.rb#L13-L15