Rails里实现多对多有两种方式:
1，has_and_belongs_to_many

# in migration
def self.up
  create_table 'categories_products', :id => false do |t|
    t.column :category_id, :integer
    t.column :product_id, :integer
  end
end

# models/product.rb
has_and_belongs_to_many :categories

# models/category.rb
has_and_belongs_to_many :products

这是最直接的多对多，用中间表categories_products来连接category和product，并且中间表里没有其他字段信息

2，has_many :through

# models/categorization.rb
belongs_to :product
belongs_to :category

# models/product.rb
has_many :categorizations
has_many :categories, :through => :categorizations

# models/category.rb
has_many :categorizations
has_many :products, :through => :categorizations

has_many :through这种方式跟habtm的不同是中间表里可以含有其余的字段信息，并且中间表在Model里明确指出