can't get p = Product.first; p.materials to work without erroring out using rails 3.2

gistfile1.rb

# see bottom for rails console example

# models/product.rb
class Product < ActiveRecord::Base
  has_many :products_materials
  has_many :materials, :through => :products_materials
end

# models/material.rb
class Material < ActiveRecord::Base
  has_many :products_materials
  has_many :products, :through => :products_materials
end

# models/products_material.rb (join model
class ProductsMaterial < ActiveRecord::Base
  belongs_to :product
  belongs_to :material
end


### migrations:
class CreateProducts < ActiveRecord::Migration
  def change
    create_table :products do |t|
      t.string :title

      t.timestamps
    end
  end
end


class CreateMaterials < ActiveRecord::Migration
  def change
    create_table :materials do |t|
      t.string :title

      t.timestamps
    end
  end
end


class CreateProductsMaterial < ActiveRecord::Migration
  def change
    create_table :products_material :id => false do |t|
      t.integer :product_id
      t.integer :material_id
    end
  end
end

Loading development environment (Rails 3.2.1)
1.9.3-p0 :002 >    p = Product.create(title: 'test prod 1')
   (0.1ms)  begin transaction
  SQL (28.8ms)  INSERT INTO "products" ("created_at", "title", "updated_at") VALUES (?, ?, ?)  [["created_at", Sun, 26 Feb 2012 01:16:56 UTC +00:00], ["title", "test prod 1"], ["updated_at", Sun, 26 Feb 2012 01:16:56 UTC +00:00]]
   (1.3ms)  commit transaction
 => #<Product id: 1, title: "test prod 1", created_at: "2012-02-26 01:16:56", updated_at: "2012-02-26 01:16:56"> 
1.9.3-p0 :003 >  m = Material.create(title: 'gold')
   (0.1ms)  begin transaction
  SQL (0.6ms)  INSERT INTO "materials" ("created_at", "title", "updated_at") VALUES (?, ?, ?)  [["created_at", Sun, 26 Feb 2012 01:17:07 UTC +00:00], ["title", "gold"], ["updated_at", Sun, 26 Feb 2012 01:17:07 UTC +00:00]]
   (2.7ms)  commit transaction
 => #<Material id: 1, title: "gold", created_at: "2012-02-26 01:17:07", updated_at: "2012-02-26 01:17:07"> 
1.9.3-p0 :004 > p.materials
NameError: uninitialized constant Product::ProductsMaterial
	from /Users/phong/.rvm/gems/ruby-1.9.3-p0@rails3.1.3/gems/activerecord-3.2.1/lib/active_record/inheritance.rb:119:in `compute_type'
	from /Users/phong/.rvm/gems/ruby-1.9.3-p0@rails3.1.3/gems/activerecord-3.2.1/lib/active_record/reflection.rb:172:in `klass'
	from /Users/phong/.rvm/gems/ruby-1.9.3-p0@rails3.1.3/gems/activerecord-3.2.1/lib/active_record/reflection.rb:385:in `block in source_reflection'
	from /Users/phong/.rvm/gems/ruby-1.9.3-p0@rails3.1.3/gems/activerecord-3.2.1/lib/active_record/reflection.rb:385:in `collect'
	from /Users/phong/.rvm/gems/ruby-1.9.3-p0@rails3.1.3/gems/activerecord-3.2.1/lib/active_record/reflection.rb:385:in `source_reflection'
	from /Users/phong/.rvm/gems/ruby-1.9.3-p0@rails3.1.3/gems/activerecord-3.2.1/lib/active_record/reflection.rb:508:in `check_validity!'
	from /Users/phong/.rvm/gems/ruby-1.9.3-p0@rails3.1.3/gems/activerecord-3.2.1/lib/active_record/associations/association.rb:26:in `initialize'
	from /Users/phong/.rvm/gems/ruby-1.9.3-p0@rails3.1.3/gems/activerecord-3.2.1/lib/active_record/associations/collection_association.rb:24:in `initialize'
	from /Users/phong/.rvm/gems/ruby-1.9.3-p0@rails3.1.3/gems/activerecord-3.2.1/lib/active_record/associations/has_many_through_association.rb:10:in `initialize'
	from /Users/phong/.rvm/gems/ruby-1.9.3-p0@rails3.1.3/gems/activerecord-3.2.1/lib/active_record/associations.rb:157:in `new'
	from /Users/phong/.rvm/gems/ruby-1.9.3-p0@rails3.1.3/gems/activerecord-3.2.1/lib/active_record/associations.rb:157:in `association'
	from /Users/phong/.rvm/gems/ruby-1.9.3-p0@rails3.1.3/gems/activerecord-3.2.1/lib/active_record/associations/builder/association.rb:44:in `block in define_readers'
	from (irb):4
	from /Users/phong/.rvm/gems/ruby-1.9.3-p0@rails3.1.3/gems/railties-3.2.1/lib/rails/commands/console.rb:47:in `start'
	from /Users/phong/.rvm/gems/ruby-1.9.3-p0@rails3.1.3/gems/railties-3.2.1/lib/rails/commands/console.rb:8:in `start'
	from /Users/phong/.rvm/gems/ruby-1.9.3-p0@rails3.1.3/gems/railties-3.2.1/lib/rails/commands.rb:41:in `<top (required)>'
	from script/rails:6:in `require'
	from script/rails:6:in `<main>'1.9.3-p0 :005 >