#47 Two Many-to-Many – RailsCasts.

Found this railscast helpful in trying to understand many to many relationships in ror. And the difference between using has_and_belongs_to_many and the has_many :through. As noted in the railscast, the has_and_belongs_to_many to very limited and does not allow as many options as has_many :through. Due to Ryan’s recommendation in the railscast, I’m going to use the has_many :through.

From  the tutorial, here’s an example of how to do many to many.

has_and_belongs_to_many example using Product and Category model (I’ve updated it to use Rails 3)

type in the command the following:

>> rails generate model category name:string

>> rails generate model product name:string

>> rails generate migration create_categories_products_join

add to CreateCategoriesProducts migration file

class CreateCategoriesProductsJoin < ActiveRecord::Migration
  def self.up
    create_table 'categories_products', :id => false do |t|
      t.column 'category_id', :integer
      t.column 'product_id', :integer
    end
  end

  def self.down
   drop_table 'categories_products'
  end
end

>> rake db:migrate
and set up the has_and_belongs_to_many associations in both the Product and Category models.
has_many :through example
Create a model at the command line
>> rails generate model categorization product_id:integer category_id:integer
*note you can add more columns to the model if needed
>>rake db:migrate
In your models add the following

class Categorization < ActiveRecord::Base
  belongs_to :product
  belongs_to :category
end
class Product < ActiveRecord::Base
  has_many :categorizations
  has_many :categories, :through => :categorizations
end

class Category < ActiveRecord::Base
  has_many :categorizations
  has_many :products, :through => :categorizations
end

Ryan ends the railscast by adding some helpful hints when trying to decide between the two. He suggests asking yourself two questions.
1)Do I need to store extra information in the join?
2)Do I need to treat the join like its own model?
If you answer yes to any of the above, he suggests using the has_many :through association. Otherwise he suggests a strong “might” to use the has_and_belongs_to_many associations. He warns that this association is very limited.