error.rb

    # Given below is snippet from ActiveRecord page.    

    #    class Dungeon < ActiveRecord::Base
    #      has_many :traps, :inverse_of => :dungeon
    #      has_one :evil_wizard, :inverse_of => :dungeon
    #    end
    #
    #    class Trap < ActiveRecord::Base
    #      belongs_to :dungeon, :inverse_of => :traps
    #    end
    #
    #    class EvilWizard < ActiveRecord::Base
    #      belongs_to :dungeon, :inverse_of => :evil_wizard
    #    end

    # There are limitations to <tt>:inverse_of</tt> support:
    #
    # * does not work with <tt>:through</tt> associations.
    # * does not work with <tt>:polymorphic</tt> associations.
    # * for +belongs_to+ associations +has_many+ inverse associations are ignored.


# This is what I did.


    create_table :dungeons do |t|
      t.string :name
    end

    create_table :traps do |t|
      t.string :name
      t.integer :dungeon_id
    end

    create_table :evil_wizards do |t|
      t.string :name
      t.integer :dungeon_id
    end


class Dungeon < ActiveRecord::Base
  has_many :traps, :inverse_of => :dungeon
  has_one  :evil_wizard, :inverse_of => :dungeon
end


class EvilWizard < ActiveRecord::Base
  belongs_to :dungeon, :inverse_of => :evil_wizard
end

class Trap < ActiveRecord::Base
  belongs_to :dungeon, :inverse_of => :trap
end

class Dungeon

  def self.cleanup
    d = Dungeon.create(:name => 'boom')
    Trap.create(:name => 'trap', :dungeon_id => d.id)
    EvilWizard.create(:name => 'dragon', :dungeon_id => d.id)
  end

  def self.has_one_test
    self.cleanup

    dungeon = Dungeon.first
    puts dungeon.object_id

    evil_wizard = EvilWizard.first
    puts evil_wizard.dungeon.object_id
  end

  def self.has_many_test
    self.cleanup

    dungeon = Dungeon.first
    puts dungeon.object_id

    trap = Trap.first
    puts trap.dungeon.object_id
  end

end

# Dungeon.has_one_test prints object_ids which are not same.
# rails runner 'Person.has_one_test'
# 2181166020
# 2181157800


# Dungeon.has_many_test blows up with following exception.
# `check_validity_of_inverse!': Could not find the inverse association for dungeon (:trap in Dungeon) # (ActiveRecord::InverseOfAssociationNotFoundError)

# Above example was tested with rails edge as of Mary 9th 2010.