Wir greifen das Beispiel aus der Rails-Doku auf (siehe http://api.rubyonrails.org/classes/ActiveRecord/Associations/ClassMethods.html) und erstellen eine Applikation, in der es Angestellte und Büros gibt. Jeder Angestellte hat ein Büro. Erst die Applikation:

MacBook:~ xyz$ rails new office-space
[...]
MacBook:~ xyz$ cd office-space 
MacBook:office-space xyz$

Und jetzt die beiden Modele:

MacBook:office-space xyz$ rails generate model employee last_name
[...]
MacBook:office-space xyz$ rails generate model office location employee_id:integer
[...]
MacBook:office-space xyz$ rake db:migrate
[...]
MacBook:office-space xyz$

Die Verknüpfung in der Datei app/model/employee.rb:

class Employee < ActiveRecord::Base
  attr_accessible :last_name

  has_one :office
end

Und das Gegenstück in der app/model/office.rb:

class Office < ActiveRecord::Base
  attr_accessible :employee_id, :location

  belongs_to :employee
end

has_one :office, :dependent => :destroy  # loescht den Office Eintrag automatisch mit
has_one :office, :dependent => :nullify  # wie :destroy, nur das der Office Datensatz nicht 
                                         # geloescht wird, sondern employee_id auf NULL gesetzt
                                         # wird
has_one :last_comment, :class_name => "Comment", :order => "posted_on"
has_one :project_manager, :class_name => "Person", :conditions => "role = 'project_manager'"
has_one :boss, :readonly => :true
has_one :club, :through => :membership
has_one :primary_address, :through => :addressables, :conditions => ["addressable.primary = ?", true], :source => :addressable

Starten wir einmal die Console und legen zwei Angestellte an:

MacBook:office-space xyz$ rails console
Loading development environment (Rails 3.2.3)
1.9.3p194 :001 > Employee
 => Employee(id: integer, last_name: string, created_at: datetime, updated_at: datetime) 
1.9.3p194 :002 > Employee.create(:last_name => 'Udelhoven')
   (0.1ms)  begin transaction
  SQL (9.3ms)  INSERT INTO "employees" ("created_at", "last_name", "updated_at") VALUES (?, ?, ?)  [["created_at", Tue, 08 May 2012 06:42:43 UTC +00:00], ["last_name", "Udelhoven"], ["updated_at", Tue, 08 May 2012 06:42:43 UTC +00:00]]
   (0.9ms)  commit transaction
 => #<Employee id: 1, last_name: "Udelhoven", created_at: "2012-05-08 06:42:43", updated_at: "2012-05-08 06:42:43"> 
1.9.3p194 :003 > Employee.create(:last_name => 'Meier')
   (0.1ms)  begin transaction
  SQL (0.5ms)  INSERT INTO "employees" ("created_at", "last_name", "updated_at") VALUES (?, ?, ?)  [["created_at", Tue, 08 May 2012 06:42:49 UTC +00:00], ["last_name", "Meier"], ["updated_at", Tue, 08 May 2012 06:42:49 UTC +00:00]]
   (5.2ms)  commit transaction
 => #<Employee id: 2, last_name: "Meier", created_at: "2012-05-08 06:42:49", updated_at: "2012-05-08 06:42:49"> 
1.9.3p194 :004 >

Jetzt bekommt ein Angestellter sein eigenes Büro:

1.9.3p194 :004 > Office.create(:location => '1. Stock', :employee_id => 1)
   (0.1ms)  begin transaction
  SQL (0.6ms)  INSERT INTO "offices" ("created_at", "employee_id", "location", "updated_at") VALUES (?, ?, ?, ?)  [["created_at", Tue, 08 May 2012 06:44:04 UTC +00:00], ["employee_id", 1], ["location", "1. Stock"], ["updated_at", Tue, 08 May 2012 06:44:04 UTC +00:00]]
   (3.3ms)  commit transaction
 => #<Office id: 1, location: "1. Stock", employee_id: 1, created_at: "2012-05-08 06:44:04", updated_at: "2012-05-08 06:44:04"> 
1.9.3p194 :005 > Employee.first.office
  Employee Load (0.3ms)  SELECT "employees".* FROM "employees" LIMIT 1
  Office Load (0.2ms)  SELECT "offices".* FROM "offices" WHERE "offices"."employee_id" = 1 LIMIT 1
 => #<Office id: 1, location: "1. Stock", employee_id: 1, created_at: "2012-05-08 06:44:04", updated_at: "2012-05-08 06:44:04"> 
1.9.3p194 :006 > Office.first.employee
  Office Load (0.3ms)  SELECT "offices".* FROM "offices" LIMIT 1
  Employee Load (0.2ms)  SELECT "employees".* FROM "employees" WHERE "employees"."id" = 1 LIMIT 1
 => #<Employee id: 1, last_name: "Udelhoven", created_at: "2012-05-08 06:42:43", updated_at: "2012-05-08 06:42:43"> 
1.9.3p194 :007 >

Beim zweiten Angestellten benutzen wir die automatisch erzeugte create_office-Methode (bei has_many würden wir hier offices.create benutzen):

1.9.3p194 :007 > Employee.last.create_office(:location => 'Erdgeschoss')
  Employee Load (0.3ms)  SELECT "employees".* FROM "employees" ORDER BY "employees"."id" DESC LIMIT 1
   (0.1ms)  begin transaction
  SQL (0.7ms)  INSERT INTO "offices" ("created_at", "employee_id", "location", "updated_at") VALUES (?, ?, ?, ?)  [["created_at", Tue, 08 May 2012 06:45:12 UTC +00:00], ["employee_id", 2], ["location", "Erdgeschoss"], ["updated_at", Tue, 08 May 2012 06:45:12 UTC +00:00]]
   (2.4ms)  commit transaction
  Office Load (0.2ms)  SELECT "offices".* FROM "offices" WHERE "offices"."employee_id" = 2 LIMIT 1
   (0.0ms)  begin transaction
   (0.0ms)  commit transaction
 => #<Office id: 2, location: "Erdgeschoss", employee_id: 2, created_at: "2012-05-08 06:45:12", updated_at: "2012-05-08 06:45:12"> 
1.9.3p194 :008 >

Löschen geht intuitiv mit destroy:

1.9.3p194 :008 > Employee.first.office
  Employee Load (0.2ms)  SELECT "employees".* FROM "employees" LIMIT 1
  Office Load (0.2ms)  SELECT "offices".* FROM "offices" WHERE "offices"."employee_id" = 1 LIMIT 1
 => #<Office id: 1, location: "1. Stock", employee_id: 1, created_at: "2012-05-08 06:44:04", updated_at: "2012-05-08 06:44:04"> 
1.9.3p194 :009 > Employee.first.office.destroy
  Employee Load (0.3ms)  SELECT "employees".* FROM "employees" LIMIT 1
  Office Load (0.2ms)  SELECT "offices".* FROM "offices" WHERE "offices"."employee_id" = 1 LIMIT 1
   (0.1ms)  begin transaction
  SQL (0.4ms)  DELETE FROM "offices" WHERE "offices"."id" = ?  [["id", 1]]
   (3.6ms)  commit transaction
 => #<Office id: 1, location: "1. Stock", employee_id: 1, created_at: "2012-05-08 06:44:04", updated_at: "2012-05-08 06:44:04"> 
1.9.3p194 :010 > Office.exists?(1)
  Office Exists (0.2ms)  SELECT 1 FROM "offices" WHERE "offices"."id" = 1 LIMIT 1
 => false 
1.9.3p194 :011 > exit
MacBook:office-space xyz$

MacBook:office-space xyz$ rails console
Loading development environment (Rails 3.2.3)
1.9.3p194 :001 > Employee.last.office
  Employee Load (0.1ms)  SELECT "employees".* FROM "employees" ORDER BY "employees"."id" DESC LIMIT 1
  Office Load (0.2ms)  SELECT "offices".* FROM "offices" WHERE "offices"."employee_id" = 2 LIMIT 1
 => #<Office id: 2, location: "Erdgeschoss", employee_id: 2, created_at: "2012-05-08 06:45:12", updated_at: "2012-05-08 06:45:12"> 
1.9.3p194 :002 > Employee.last.create_office(:location => 'Keller')
  Employee Load (0.3ms)  SELECT "employees".* FROM "employees" ORDER BY "employees"."id" DESC LIMIT 1
   (0.1ms)  begin transaction
  SQL (29.3ms)  INSERT INTO "offices" ("created_at", "employee_id", "location", "updated_at") VALUES (?, ?, ?, ?)  [["created_at", Tue, 08 May 2012 06:51:58 UTC +00:00], ["employee_id", 2], ["location", "Keller"], ["updated_at", Tue, 08 May 2012 06:51:58 UTC +00:00]]
   (3.7ms)  commit transaction
  Office Load (0.2ms)  SELECT "offices".* FROM "offices" WHERE "offices"."employee_id" = 2 LIMIT 1
   (0.0ms)  begin transaction
   (0.3ms)  UPDATE "offices" SET "employee_id" = NULL, "updated_at" = '2012-05-08 06:51:58.969829' WHERE "offices"."id" = 2
   (0.9ms)  commit transaction
 => #<Office id: 3, location: "Keller", employee_id: 2, created_at: "2012-05-08 06:51:58", updated_at: "2012-05-08 06:51:58"> 
1.9.3p194 :003 > Employee.last.office
  Employee Load (0.3ms)  SELECT "employees".* FROM "employees" ORDER BY "employees"."id" DESC LIMIT 1
  Office Load (0.2ms)  SELECT "offices".* FROM "offices" WHERE "offices"."employee_id" = 2 LIMIT 1
 => #<Office id: 3, location: "Keller", employee_id: 2, created_at: "2012-05-08 06:51:58", updated_at: "2012-05-08 06:51:58"> 
1.9.3p194 :004 >

1.9.3p194 :004 > Office.all
  Office Load (0.2ms)  SELECT "offices".* FROM "offices" 
 => [#<Office id: 2, location: "Erdgeschoss", employee_id: nil, created_at: "2012-05-08 06:45:12", updated_at: "2012-05-08 06:51:58">, #<Office id: 3, location: "Keller", employee_id: 2, created_at: "2012-05-08 06:51:58", updated_at: "2012-05-08 06:51:58">] 
1.9.3p194 :005 > exit
MacBook:office-space xyz$

Sowohl has_one wie auch belongs_to bietet die Möglichkeit, eine 1:1-Verbindung abzubilden. Der Unterschied liegt in der Praxis in der persönlichen Präferenz des Programmierers und dem Ort des Foreign-Keys. Im Allgemeinen wird has_one eher selten benutzt und hängt vom Normalisierungsgrad des Datenschemas ab.