Matt on Rails

If Ruby5 can have tales of drunkenness, so can my ruby blog.

On Sunday, on the way back home from town, we were driving down a four-lane city street. The guy just ahead of us in the other lane was swerving towards oncoming traffic. Every time a car would come the other way, he would drift across the yellow line. Anticipating a collision that would send his truck rebounding into my lane, I slowed and left plenty of space. This foresight proved prudent when the road narrowed to two lanes, as he swerved at the last second into my lane.

I continued to leave plenty of space, about 150 yards, which brought out the impatience in the drivers behind me. Two such drivers passed on the now dark country highway, only to find obviously-inebriated-dude just ahead. The driver-who-shouldn’t-have-been pulled part way off the road and stopped (on a state highway!) to let the rest of us pass. I tried to slow down enough that he’d go on without me passing, but he waved me on. (This was the wave that is featured in the title of this post.)

I reluctantly passed, and traffic ahead of me slowed. I slowed, and saw the lights behind me accelerate. Great, I thought, now I’m going to get rear-ended. I pulsed my brake lights, and that got his attention so he slowed. The cars ahead sped up, then slowed again. Repeat the pulsing lights to encourage Mr. Tipsy to slow down. At the first side street, I turned off the highway, turned around, and got back on the highway. This was plenty of time for the subject of the story to pass.

At the next major intersection, I was committed to turning a different direction from the other traveller. He went straight, down a gravel county road (county road 50 S), which suited me fine, as I was turning right (onto highway 29).

The rest of the story made the paper. Fortunately, nobody suffered permanent injuries.

This is a not that common of a problem, though there are other posts on the topic. But my solution’s a little different from what I found, so I figured it was blog-worthy. I used this in a rails 3.0 app. I’ve also only tried it with sqlite, so it may totally blow up when I push to heroku. YMMV.

Take, for example, some initial tables:

users – id:primary_key
replies – id:primary_key
liked_replies – user_id:integer, reply_id:integer

and some initial models:

class User < ActiveRecord::Base
  has_and_belongs_to_many :liked_replies, :class_name => 'Reply', :join_table => 'liked_replies'
end

class Reply < ActiveRecord::Base
  has_and_belongs_to_many :likers, :class_name => 'User', :join_table => 'liked_replies'
end

And, of course, there’s existing data that you don’t want to destroy.

So, I generated a migration:

bundle exec rails g migration add_id_to_liked_replies id:primary_key

If you’re curious, the migration looks like this:

class AddIdToLikedReplies < ActiveRecord::Migration
  def self.up
    add_column :liked_replies, :id, :primary_key
  end
  def self.down
    remove_column :liked_replies, :id
  end
end

Migrate as usual, and then the app stops running, because routes can’t be built, because User is a devise model, and devise_for tries to load User which has an association that generates one of these:

Primary key is not allowed in a has_and_belongs_to_many join table (liked_replies). (ActiveRecord::HasAndBelongsToManyAssociationWithPrimaryKeyError)

Hm. This will probably happen when we deploy, too. And this makes a db:rollback fail, too. !!!! So, how can we make the code work with or without a primary key? I’ve seen lots of people talk about making your code work with the before & after version of your db, and that seems like the right goal here, too.

Here are my new model classes that work with either the new or old liked_replies table:

class User < ActiveRecord::Base
  begin
    has_and_belongs_to_many :liked_replies, :class_name => 'Reply', :join_table => 'liked_replies'
  rescue
    has_many :liked_reply_records, :class_name => 'LikedReply', :dependent => :destroy
    has_many :liked_replies, :through => :liked_reply_records, :source => :reply
  end
end

class Reply < ActiveRecord::Base
  begin
    has_and_belongs_to_many :likers, :class_name => 'User', :join_table => 'liked_replies'
  rescue
    has_many :liked_replies
    has_many :likers, :through => :liked_replies, :source => :user
  end
end

class LikedReply < ActiveRecord::Base
  belongs_to :user
  belongs_to :reply
end