class Category < ActiveRecord::Base
  has_many :item_types
  has_many :items, :through => :item_types, :source => :category
end

class ItemType < ActiveRecord::Base
  belongs_to :category
  has_many :items
end

class Item
  belongs_to :item_type
end

Category.joins(:item_types,:items).where("category.id=?",1)

Category.joins(:item_types, :items).where(:item_type => {:category_id => 1})

# app/models/category.rb
class Category < ActiveRecord::Base
  has_many :item_types
  has_many :items, :through => :item_types
end

# app/models/item_type.rb
class ItemType < ActiveRecord::Base
  belongs_to :category
  belongs_to :item
end

# app/models/item.rb
class Item < ActiveRecord::Base
  has_many :item_types
  has_many :categories, :through => :item_types
end

# app/controllers/categories_controller.rb
def show
  @category = Category.find(params[:id])
  @category_items = @category.items
end