class Project < ActiveRecord::Base
    belongs_to :assignment
    belongs_to :programmer
end

class Programmer < ActiveRecord::Base
    has_many :projects
    has_many :assignments, :through => :projects
end

class Assignment < ActiveRecord::Base
    has_many :projects
    has_many :programmers, :through => :projects
end

p = Programmer.create(:name => "Mike")

p.assignments.create(:name => "homework4")

p.assignments[0] = Assignment.find_or_create_by_name("homework1")

p.save

SELECT "assignments".* FROM "assignments" WHERE "assignments"."name" = 'homework1' LIMIT 1

proj = p.projects.first

proj.assignment_id = 12
proj.save!

def self.up
  create_table :configurations do |t|
    t.string :name
    t.references :project  # This store just the id of the object.
    t.timestamps
  end
end

rake db:migrate