Relations in Ruby

employee.rb

require 'relations'

class Employee
  extend Relations

  belongs_to :manager
end

employee_spec.rb

require 'manager'
require 'employee'

describe Employee do
  let(:manager) { Manager.new }

  it 'belongs to a manager' do
    subject.manager.should be_nil

    subject.manager = manager

    subject.manager.should == manager
  end

  it "adds itself to the manager's list of employees" do
    manager.employees.should be_empty

    subject.manager = manager

    manager.employees.should include subject
  end
end

manager.rb

require 'relations'

class Manager
  extend Relations

  has_many :employees
end

manager_spec.rb

require 'employee'
require 'manager'

describe Manager do
  let(:employee) { Employee.new }

  it 'has many employees' do
    subject.employees.should be_empty

    subject.employees << employee

    subject.employees.size.should == 1
    subject.employees.should include employee
  end

  it "updates the employee's manager" do
    employee.manager.should be_nil

    subject.employees << employee

    employee.manager.should == subject
  end
end

relations.rb

module Relations
  require 'active_support/inflector'

  class RelationCollection < Array
    def initialize(parent, parent_name, child_name)
      @parent      = parent
      @parent_name = parent_name
      @child_name  = child_name
    end

    def <<(child)
      unless child.send(@parent_name) == @parent
        child.send "#{@parent_name}=", @parent
      end
      super unless include?(child)
    end
  end

  def belongs_to(parent)
    send(:attr_reader, parent)

    define_method "#{parent}=" do |p|
      instance_variable_set("@#{parent}", p)
      p.send(self.class.name.underscore.pluralize) << self
    end
  end

  def has_many(children)
    define_method children do
      collection = instance_variable_get "@#{children}"

      unless collection
        parent_name = self.class.name.underscore
        collection = RelationCollection.new(self, parent_name, children)
        instance_variable_set "@#{children}", collection
      end

      collection
    end
  end
end