ruby sinatra dm-sqlite-adapter (LoadError)

rubygems/custom_require.rb:36:in `require': cannot load such file -- dm-sqlite-adapter (LoadError)
require 'rubygems'
require 'sinatra'
require 'data_mapper' # metagem, requires common plugins too.
# need install dm-sqlite-adapter
DataMapper::setup(:default, "sqlite3://#{Dir.pwd}/blog.db")

    class Post
     include DataMapper::Resource
     property :id, Serial
     property :title, String
     property :body, Text
     property :created_at, DateTime
     end
 
 # Perform basic sanity checks and initialize all relationships
 # Call this when you've defined all your models
     DataMapper.finalize
 
 # automatically create the post table
     Post.auto_upgrade!
 

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