gistfile1.rb

1.9.3-p0 :001 > p = Product.create(title: "Test Product 1")
   (0.1ms)  begin transaction
  SQL (62.0ms)  INSERT INTO "products" ("created_at", "title", "updated_at") VALUES (?, ?, ?)  [["created_at", Sun, 26 Feb 2012 01:41:35 UTC +00:00], ["title", "Test Product 1"], ["updated_at", Sun, 26 Feb 2012 01:41:35 UTC +00:00]]
   (23.4ms)  commit transaction
 => #<Product id: 1, title: "Test Product 1", created_at: "2012-02-26 01:41:35", updated_at: "2012-02-26 01:41:35"> 
1.9.3-p0 :002 > m = Material.create(title: "Iron")
   (0.1ms)  begin transaction
  SQL (0.7ms)  INSERT INTO "materials" ("created_at", "title", "updated_at") VALUES (?, ?, ?)  [["created_at", Sun, 26 Feb 2012 01:41:43 UTC +00:00], ["title", "Iron"], ["updated_at", Sun, 26 Feb 2012 01:41:43 UTC +00:00]]
   (44.0ms)  commit transaction
 => #<Material id: 1, title: "Iron", created_at: "2012-02-26 01:41:43", updated_at: "2012-02-26 01:41:43"> 
1.9.3-p0 :003 > p.materials << m

ActiveRecord::HasManyThroughAssociationNotFoundError: Could not find the association :products_materials in model Product