Setting up a polymorphic has_many :through relationship

Question

rails g model Article name:string
rails g model Category name:string
rails g model Tag name:string taggable_id:integer taggable_type:string category_id:integer

I have created my models as shown in the preceding code. Articles will be one of many models which can have tags. The category model will contain all categories which may be assigned. The tag model will be a polymorphic join-table which represents tagged relationships.

class Article < ActiveRecord::Base
  has_many :tags, :as => :taggable
  has_many :categories, :through => :taggable
end

class Category < ActiveRecord::Base
  has_many :tags, :as => :taggable
  has_many :articles, :through => :taggable
end

class Tag < ActiveRecord::Base
  belongs_to :taggable, :polymorphic => true
  belongs_to :category
end

I can't seem to get this to work, I can do it non polymorphic, but I must have something wrong with the polymorphic part. Any ideas?

Edit: Still not getting this right:

class Article < ActiveRecord::Base
    has_many :taggables, :as => :tag
    has_many :categories, :through => :taggables, :source => :tag, :source_type => "Article"
end
class Category < ActiveRecord::Base
    has_many :taggables, :as => :tag
    has_many :articles, :through => :taggables, :source => :tag, :source_type => "Article"
end
class Tag < ActiveRecord::Base
  belongs_to :taggable, :polymorphic => true
  belongs_to :category
end

Answer 1

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.