Asociaciones Simples

• Associations are a way of declaring relationships between models, for example a blog Post "has many" Comments, or a Comment belongs to a Post.

• Associations add a series of methods to your models which allow you to create relationships and retrieve related models along with a few other useful features.

• Which records are related to which are determined by their foreign keys (cláves foráneas).

The types of associations currently in DataMapper are:

has n  
has 1 
belongs_to 
has n, :things, :through => Resource   
has n, :things, :through => :model

Este código se encuentra en: app.rb

[~/rubytesting/database/datamapper/two(master)]$ cat app.rb 
require 'data_mapper'

DataMapper::Logger.new($stdout, :debug)
DataMapper.setup(:default, "sqlite3://#{Dir.pwd}/example.db")

class Post
  include DataMapper::Resource

  property :id, Serial
  property :text, Text

  has n, :comments
end

class Comment
  include DataMapper::Resource

  property :id,     Serial
  property :text, Text

  belongs_to :post  # defaults to :required => true

end

DataMapper.finalize

require  'dm-migrations'

#DataMapper.auto_migrate!
DataMapper.auto_upgrade!

The belongs_to method accepts a few options.

• belongs_to relationships will be required by default

  You can make the parent resource optional by passing :required => false as an option to belongs_to.

• If the relationship makes up (part of) the key of a model, you can tell DataMapper to include it as part of the primary key by adding the :key => true option.

Arrancamos Pry

[~/rubytesting/database/datamapper/two]$ pry

y cargamos ./app.rb

[1] pry(main)> load './app.rb'
 ~ (0.000397) PRAGMA table_info("posts")
 ~ (0.000019) SELECT sqlite_version(*)
 ~ (0.001237) CREATE TABLE "posts" ("id" INTEGER NOT NULL PRIMARY KEY AUTOINCREMENT, "text" TEXT)
 ~ (0.000011) PRAGMA table_info("comments")
 ~ (0.000986) CREATE TABLE "comments" ("id" INTEGER NOT NULL PRIMARY KEY AUTOINCREMENT, "text" TEXT, "post_id" INTEGER NOT NULL)
 ~ (0.000964) CREATE INDEX "index_comments_post" ON "comments" ("post_id")
=> true

Este es el esquema de la base de datos creada a partir de las clases Post y Comment:

[~/rubytesting/database/datamapper/two(master)]$ sqlite3 example.db 
SQLite version 3.7.11 2012-03-20 11:35:50
Enter ".help" for instructions
Enter SQL statements terminated with a ";"
sqlite> .schema
CREATE TABLE "comments" ("id" INTEGER NOT NULL PRIMARY KEY AUTOINCREMENT, 
                         "text" TEXT, 
                         "post_id" INTEGER NOT NULL);
CREATE TABLE "posts" ("id" INTEGER NOT NULL PRIMARY KEY AUTOINCREMENT, 
                      "text" TEXT);
CREATE INDEX "index_comments_post" ON "comments" ("post_id");

Ahora podemos crear un objeto Post:

[2] pry(main)> pp = Post.create(:text => 'hello world!')
 ~ (0.000888) INSERT INTO "posts" ("text") VALUES ('hello world!')
=> #<Post @id=1 @text="hello world!">

Sus comments están vacíos:

[3] pry(main)> pp.comments
 ~ (0.000051) SELECT "id", "post_id" FROM "comments" WHERE "post_id" = 1 ORDER BY "id"
=> []

Adding resources to a relationship, can be done in different ways.

• [2] pry(main)> co = Comment.new(:text => "Nice greeting!")
  => #<Comment @id=nil @text="Nice greeting!" @post_id=nil>
  [3] pry(main)> pp = Post.create(:text => 'hello world!')
   ~ (0.001294) INSERT INTO "posts" ("text") VALUES ('hello world!')
   => #<Post @id=1 @text="hello world!">
  [4] pry(main)> co.post=pp
   => #<Post @id=1 @text="hello world!">
  [5] pry(main)> co.save
  ~ (0.002974) INSERT INTO "comments" ("text", "post_id") VALUES ('Nice greeting!', 1)
  => true
  [7] pry(main)> pp.comments
  ~ (0.000074) SELECT "id", "post_id" FROM "comments" WHERE "post_id" = 1 ORDER BY "id"
  => [#<Comment @id=1 @text=<not loaded> @post_id=1>]
  [8] pry(main)> pp.comments[0].text
   ~ (0.000193) SELECT "id", "text" FROM "comments" WHERE "id" = 1 ORDER BY "id"
   => "Nice greeting!"
  

• belongs_to relationships will be required by default

  We can make the parent resource optional by passing :required => false as an option to belongs_to.

  [12] pry(main)> c2 = Comment.create(:text => 'silly comment')
  => #<Comment @id=nil @text="silly comment" @post_id=nil>
  [13] pry(main)> c2.save
  => false
  [14] pry(main)> c2.post = pp
  => #<Post @id=1 @text="hello world!">
  [15] pry(main)> c2.save
   ~ (0.001910) INSERT INTO "comments" ("text", "post_id") VALUES ('silly comment', 1)
   => true
  

• [6] pry(main)> pp.comments.push(Comment.create(:text => "Nice greeting!"))
  => [#<Comment @id=nil @text="Nice greeting!" @post_id=1>]
  [4] pry(main)> pp
  => #<Post @id=1 @text="hello world!">
  [5] pry(main)> pp.comments
  => [#<Comment @id=nil @text="Nice greeting!" @post_id=1>]
  

  Esto no ha guardado pp en la base de datos. Ha sido modificado en memoria. Tampoco se ha guardado en la base de datos el objeto comentario.

  [7] pry(main)> pp.save
   ~ (0.002523) INSERT INTO "comments" ("text", "post_id") VALUES ('Nice greeting!', 1)
  => true
  

  Ahora las tablas contienen:

  sqlite> select * from posts;
  1|hello world!
  sqlite> select * from comments;
  1|Nice greeting!|1
  

• [8] pry(main)> c2 = Comment.create(:text => "Do we know each other?")
  => #<Comment @id=nil @text="Do we know each other?" @post_id=nil>
  [9] pry(main)> c2.post= pp
  => #<Post @id=1 @text="hello world!">
  [10] pry(main)> c2
  => #<Comment @id=nil @text="Do we know each other?" @post_id=1>
  [11] pry(main)> c2.save
   ~ (0.007430) INSERT INTO "comments" ("text", "post_id") VALUES ('Do we know each other?', 1)
  => true
  [12] pry(main)> pp.comments
  => [#<Comment @id=1 @text="Nice greeting!" @post_id=1>]
  

  [16] pry(main)> aa = Post.all
   ~ (0.000065) SELECT "id" FROM "posts" ORDER BY "id"
  => [#<Post @id=1 @text=<not loaded>>]
  [17] pry(main)> aa[0].text
   ~ (0.000052) SELECT "id", "text" FROM "posts" WHERE "id" = 1 ORDER BY "id"
  => "hello world!"
  [20] pry(main)> aa[0].comments
   ~ (0.000052) SELECT "id", "post_id" FROM "comments" WHERE "post_id" = 1 ORDER BY "id"
  => [#<Comment @id=1 @text=<not loaded> @post_id=1>, 
      #<Comment @id=2 @text=<not loaded> @post_id=1>]