I struggled a bit this afternoon getting Datamapper’s dm-is-remixable plug-in as there aren’t too many posts out there and some are out of date. Hopefully this quick overview will spare you my pain 
Many sites allow you to comment on blog posts, images, videos, status, etc. having one comments table with post_id, image_id, etc. as with a One-to-Many relationship leads to a giant table of unrelated (other than by user_id) data. In my opinion it’s better to have a table for each e.g. post_comments, image_comments, etc. Enter dm-is-remixable.
Step 1: Install
We’ll install dm-is-remixable and friends. Open up a terminal and enter:
sudo gem install do_sqlite3
sudo gem install datamapper
sudo gem install dm-migrations
sudo gem install dm-is-remixable
Step 2: Setup
Create a new file called remixable.rb we’ll user this file from now on. First enter the required gems and set Datamapper’s log and sqlite file.
require 'rubygems'
require 'dm-core'
require 'dm-migrations'
require 'dm-is-remixable'
# setup
DataMapper::Logger.new($stdout, :debug)
DataMapper.setup(:default, "sqlite3://#{Dir.pwd}/remixable.db")
Step 3 Define the Models
We’ll create four models, one module and three classes. User, Post, and Image are classes, and Comment will be a module. The relationships are a user has many posts and images, posts and images belong to a user, nothing new yet, and post and image remix many comments for a user. What? This is the same as saying a post has many comments and said comment(s) belong to a user.
# module and classes
module Comment
include DataMapper::Resource
property :id, Serial
property :comment, Text
is :remixable
end
class User
include DataMapper::Resource
property :id, Serial
property :username, String
has n, :images
has n, :posts
end
class Post
include DataMapper::Resource
property :id, Serial
property :title, String
property :body, Text
belongs_to :user
remix n, :comments, :for => 'User'
end
class Image
include DataMapper::Resource
property :id, Serial
property :path, String
belongs_to :user
remix n, :comments, :for => 'User'
end
Step 4: Migrate
Next we’ll finalize our models and run auto_migrate to create the tables. During this step Datamapper, via is remixable, will generate two anonymous model classes PostComment and ImageComment. Five tables will be created users, posts, images, post_comments, and image_comments.
# lock and load
DataMapper.finalize
DataMapper.auto_migrate!
Step 5: Do Something
The last step is to create some dummy data to test out our models. We’ll create two users ‘foo’ and ‘funk’, foo will create post and ‘image’ and ‘funk’ will comment on them. The last two lines show how to access post and image comments for a user.
# go!
foo = User.create(
:username => "foo"
)
funk = User.create(
:username => "funk"
)
post = Post.create(
:user_id => foo.id,
:title => "My great post",
:body => "This is my great post"
)
post_comment = PostComment.create(
:user_id => funk.id,
:post_id => post.id,
:comment => "ballz I say!"
)
image = Image.create(
:user_id => foo.id,
:path => "/some/image/path"
)
image_comment = ImageComment.create(
:user_id => funk.id,
:image_id => image.id,
:comment => "you look funny!"
)
puts funk.post_comments[0].comment
puts funk.image_comments[0].comment
That’s it!
