Rails3 and Datamapper

I am going to migrate from rails 3 app, that used AR and arel to datamapper. I love chains of scopes like Person.where(...).where(...).somescope.paginate(...).order(开发者_C百科...). how to migrate from this arel approach to datamapper.

Named scopes in DataMapper are simply class methods that you define on your model classes. Inside those class methods, you typically call #all with some conditions to get back a DataMapper::Collection. In order for your class methods to be chainable, you must make sure to return a DataMapper::Collection.

For completeness' sake, here are the installation instructions ...

gem install dm-core
gem install dm-migrations
gem install dm-sqlite-adapter
gem install dm-chunked_query

And the code to get you going (put this into test.rb for maximum reproducability)

require 'rubygems'
require 'dm-core'
require 'dm-migrations'
require 'dm-chunked_query'

class Person
  include DataMapper::Resource

  property :id,        Serial
  property :name,      String
  property :hobby,     String
  property :country,   String
  property :continent, String

  def self.european
    all(:continent => 'Europe')
  end

  def self.hackers
    all(:hobby => 'Hacking')
  end

  def self.named(name)
    all(:name => name)
  end
end

DataMapper::Logger.new($stdout, :debug)
DataMapper.setup(:default, 'sqlite::memory:')

DataMapper.finalize.auto_migrate!

1.upto(10) do |i|
  Person.create(:name => "Alex", :hobby => 'Hacking', :country => "Country-#{i}", :continent => 'Europe')
end

# you could even skip the explicit call to #all
# i just left it in there because it reads nicely
Person.all.european.hackers.named('Alex').chunks(5).each_with_index do |chunk, idx|
  puts "Rendering page #{idx + 1}"
  chunk.each do |person|
    puts "Someone named #{person.name} who lives in #{person.country}"
  end
end

__END__

ruby-1.9.2-p180@datamapper mungo:dm-rails snusnu$ ruby test.rb
 ~ (0.000102) SELECT sqlite_version(*)
 ~ (0.000132) DROP TABLE IF EXISTS "people"
 ~ (0.000013) PRAGMA table_info("people")
 ~ (0.000315) CREATE TABLE "people" ("id" INTEGER NOT NULL PRIMARY KEY AUTOINCREMENT, "name" VARCHAR(50), "hobby" VARCHAR(50), "country" VARCHAR(50), "continent" VARCHAR(50))
 ~ (0.000049) INSERT INTO "people" ("name", "hobby", "country", "continent") VALUES ('Alex', 'Hacking', 'Country-1', 'Europe')
 ~ (0.000056) INSERT INTO "people" ("name", "hobby", "country", "continent") VALUES ('Alex', 'Hacking', 'Country-2', 'Europe')
 ~ (0.000044) INSERT INTO "people" ("name", "hobby", "country", "continent") VALUES ('Alex', 'Hacking', 'Country-3', 'Europe')
 ~ (0.000043) INSERT INTO "people" ("name", "hobby", "country", "continent") VALUES ('Alex', 'Hacking', 'Country-4', 'Europe')
 ~ (0.000037) INSERT INTO "people" ("name", "hobby", "country", "continent") VALUES ('Alex', 'Hacking', 'Country-5', 'Europe')
 ~ (0.000038) INSERT INTO "people" ("name", "hobby", "country", "continent") VALUES ('Alex', 'Hacking', 'Country-6', 'Europe')
 ~ (0.000035) INSERT INTO "people" ("name", "hobby", "country", "continent") VALUES ('Alex', 'Hacking', 'Country-7', 'Europe')
 ~ (0.000035) INSERT INTO "people" ("name", "hobby", "country", "continent") VALUES ('Alex', 'Hacking', 'Country-8', 'Europe')
 ~ (0.000036) INSERT INTO "people" ("name", "hobby", "country", "continent") VALUES ('Alex', 'Hacking', 'Country-9', 'Europe')
 ~ (0.000039) INSERT INTO "people" ("name", "hobby", "country", "continent") VALUES ('Alex', 'Hacking', 'Country-10', 'Europe')
 ~ (0.000069) SELECT "id", "name", "hobby", "country", "continent" FROM "people" WHERE ("continent" = 'Europe' AND "hobby" = 'Hacking' AND "name" = 'Alex') ORDER BY "id"
Rendering page 1
Someone named Alex who lives in Country-1
Someone named Alex who lives in Country-2
Someone named Alex who lives in Country-3
Someone named Alex who lives in Country-4
Someone named Alex who lives in Country-5
Rendering page 2
Someone named Alex who lives in Country-6
Someone named Alex who lives in Country-7
Someone named Alex who lives in Country-8
Someone named Alex who lives in Country-9
Someone named Alex who lives in Country-10

See https://github.com/postmodern/dm-chunked_query for more information on the lowlevel approach to pagination (batch processing) that provides the #chunks method used in this example.