To create a polymorphic has_many :through, you must first create your models. We will use'Article,' 'Category,' and 'Tag' where 'Tag' is the join-model and Article is one of many objects which can be "tagged" with a category.
First you create your 'Article' and 'Category' models. These are basic models which do not need any special attention, just yet:
rails g model Article name:string
rails g model Category name:string
Now, we will create our polymorphic join-table:
rails g model Tag taggable_id:integer taggable_type:string category_id:integer
The join-table joins together two tables, or in our case one table to many others via polymorphic behavior. It does this by storing the ID from two separate tables. This creates a link. Our 'Category' table will always be a 'Category' so we include 'category_id.' The tables it links to vary, so we add an item 'taggable_id' which holds the id of any taggable item. Then, we use 'taggable_type' to complete the link allowing the link to know what it is linked to, such as an article.
Now, we need to set up our models:
class Article < ActiveRecord::Base
has_many :tags, :as => :taggable, :dependent => :destroy
has_many :categories, :through => :tags
end
class Category < ActiveRecord::Base
has_many :tags, :dependent => :destroy
has_many :articles, :through => :tags, :source => :taggable, :source_type => 'Article'
end
class Tag < ActiveRecord::Base
belongs_to :taggable, :polymorphic => true
belongs_to :category
end
After this, setup your database using:
rake db:migrate
That's it! Now, you can setup your database with real data:
Category.create :name => "Food"
Article.create :name => "Picking the right restaurant."
Article.create :name => "The perfect cherry pie!"
Article.create :name => "Foods to avoid when in a hurry!"
Category.create :name => "Kitchen"
Article.create :name => "The buyers guide to great refrigeration units."
Article.create :name => "The best stove for your money."
Category.create :name => "Beverages"
Article.create :name => "How to: Make your own soda."
Article.create :name => "How to: Fermenting fruit."
Now you have a few categories and various articles. They are not categorized using tags, however. So, we will need to do that:
a = Tag.new
a.taggable = Article.find_by_name("Picking the right restaurant.")
a.category = Category.find_by_name("Food")
a.save
You could then repeat this for each, this will link your categories and articles. After doing this you will be able to access each article's categories and each categorie's articles:
Article.first.categories
Category.first.articles
Notes:
1)Whenever you want to delete an item that is linked by a link-model make sure to use "destroy." When you destroy a linked object, it will also destroy the link. This ensures that there are no bad or dead links. This is why we use ':dependent => :destroy'
2)When setting up our 'Article' model, which is one our 'taggable' models, it must be linked using :as. Since in the preceeding example we used 'taggable_type' and 'taggable_id' we use :as => :taggable. This helps rails know how to store the values in the database.
3)When linking categories to articles, we use:
has_many :articles, :through => :tags, :source => :taggable, :source_type => 'Article'
This tells the category model that it should have many :articles through :tags. The source is :taggable, for the same reason as above. The source-type is "Article" because a model will automatically set taggable_type to its own name.
