ruby on railsでコンポジットパターン

bodies.yml

# test/fixtures/bodies.yml
one:
  id: 1
  file_entry_id: 1
  description: desc1

two:
  id: 2
  file_entry_id: 3
  description: description2

three:
  id: 3
  file_entry_id: 5
  description: hello, world

body.rb

# app/models/body.rb
class Body < ActiveRecord::Base
  attr_accessible :description
  belongs_to :file_entry
  def size
    description.bytesize
  end
end

chid_item.rb

# app/models/chid_item.rb
class ChildItem < ActiveRecord::Base
  attr_accessible :directory_entry_id, :entry_id
  belongs_to :entry
  belongs_to :directory_entry
end

child_items.yml

# test/fixtures/child_items.yml
one:
  id: 1
  entry_id: 1
  directory_entry_id: 2

two:
  id: 2
  entry_id: 3
  directory_entry_id: 2

three:
  id: 3
  entry_id: 5
  directory_entry_id: 4

four:
  id: 4
  entry_id: 2
  directory_entry_id: 4

directory_entry.rb

# app/models/directory_entry.
class DirectoryEntry < Entry
  has_many :child_items
  has_many :entries, through: :child_items
  def size
    entries.inject(0) {|size, entry| size += entry.size; size}
  end
end

entries.yml

# test/fixtures/entries.yml
one:
  id: 1
  name: file_name
  type: FileEntry

two:
  id: 2
  name: directory_name
  type: DirectoryEntry
three:
  id: 3
  name: file_name2
  type: FileEntry
four:
  id: 4
  name: directory_name2
  type: DirectoryEntry
five:
  id: 5
  name: file_name3
  type: FileEntry

entry.rb

# app/models/entry.rb
class Entry < ActiveRecord::Base
  attr_accessible :name, :type
end

entry_test.rb

# test/unit/entry_test.rb
require 'test_helper'

class EntryTest < ActiveSupport::TestCase
  test "file_entry has body" do
    body = entries(:one).body
    assert_equal "desc1", body.description
  end
  test "directory_entry has entries" do
    entries = entries(:two).entries
    assert_equal 2, entries.count
    assert_equal "file_name", entries[0].name
    assert_equal "file_name2", entries[1].name
  end

  test "entry size returns byte size of description" do
    assert_equal 5, entries(:one).size
    assert_equal 17, entries(:two).size
    assert_equal 29, entries(:four).size
  end
end

file_entry.rb

# app/models/file_entry.rb
class FileEntry < Entry
  has_one :body
  def size
    body.size
  end
end

schema.rb

# db/schema.rb
ActiveRecord::Schema.define(:version => 20130627094453) do

  create_table "bodies", :force => true do |t|
    t.text     "description"
    t.integer  "file_entry_id"
    t.datetime "created_at",    :null => false
    t.datetime "updated_at",    :null => false
  end

  create_table "child_items", :force => true do |t|
    t.integer  "entry_id"
    t.integer  "directory_entry_id"
    t.datetime "created_at",         :null => false
    t.datetime "updated_at",         :null => false
  end

  create_table "entries", :force => true do |t|
    t.string   "name"
    t.string   "type"
    t.datetime "created_at", :null => false
    t.datetime "updated_at", :null => false
  end

end