Rails3 et Datamapper

Question

Je vais migrer de l'application rails 3 qui utilisait AR et arel pour datamapper. J'adore les chaînes de portées comme Person.where (...). Où (...). Somescope.paginate (...). Order (...). comment migrer de cette approche arel vers datamapper.

Answer 1

Les étendues nommées dans DataMapper sont simplement des méthodes de classe que vous définissez sur vos classes de modèle. Dans ces méthodes de classe, vous appelez généralement #all avec certaines conditions pour obtenir un DataMapper::Collection. Pour que vos méthodes de classe soient chaînables, vous devez assurez-vous de renvoyer un DataMapper::Collection.

Pour être complet bien, voici les instructions d'installation ...

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

Et le code pour vous aider à aller (mettre cela en test.rb pour un maximum reproductibilité)

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 

Voir https://github.com/postmodern/dm-chunked_query pour plus d'informations sur l'approche de bas niveau de pagination (traitement par lots) qui fournit la méthode #chunks utilisée dans cet exemple.