# ActiveRecord Polymorphism

## Objectives

* Define polymorphism (in general)
* Describe the purpose of polymorphic associations
* Use the Rails model generator to generate a polymorphic model
* Implement polymorphic associations

## What are Polymorphic Associations?

To understand what polymorphic associations are, let's understand the first word, `polymorphism`.

> In object-oriented programming, polymorphism is the provision of a single interface to entities of different types.

In the context of Rails models, polymorphic associations allow a **single model** to reference **multiple models** via an association.

Sometimes a model needs to be able to reference multiple models. For example, a model to hold a vote could belong to a post and a comment (like on Reddit). While we could organize our models to include an id for each model, like this:

* Vote
  * user\_id
  * value
  * post\_id
  * comment\_id

...this model has a couple key issues.

1. A vote ideally only belongs to a post or comment, not both at once. Therefore, if the vote belonged to a post, the comment attribute would be set to NULL. This is a waste of space!
2. If we wanted a vote to reference another model (such as a message), we'd have to add another column for the model. This results in having to create another migration, and now the problem we had in issue #1 is multiplied.

Polymorphism solves this problem by abstracting the the "multiple models" into two columns.

* Vote
  * user\_id
  * value
  * votable\_type
  * votable\_id

Instead of giving the model attribute a specific name, we store the name of the model and the id as attributes. Now, `votable_type` can be any model we choose, and `votable_id` will be the `id` of the `votable_type`. Here's some examples:

* Vote 1
  * user\_id: 3
  * value: -1
  * votable\_id: 4
  * votable\_type: 'Post'
* Vote 2
  * user\_id: 9
  * value: 1
  * votable\_id: 8
  * votable\_type: 'Comment'

Let's setup a vote model in order to implement this functionality in our Link Board.

### Setup the Vote Model

```
rails g model vote value:integer user:references votable:references{polymorphic}
```

Here, we're creating a model that has a value, a user that created the vote, and a polymorphic association called `votable`. We're going to use `votable` to associate a vote to posts and comments.

**Check the migration**

make sure the vote migration has `polymorphic:true`

```ruby
t.references :votable, polymorphic:true, index: true
```

**run migration**

```
rake db:migrate
```

Now that we're migrated, we'll be adding the associations to posts and users. First thing's first, let's double-check that the vote model has a polymorphic association:

**models/vote.rb**

```ruby
belongs_to :votable, polymorphic: true
```

Now, let's associate votes to posts. We're going to define the attribute name as the plural of vote, which is `votes`. We'll define the association through the polymorphic attributes set up in vote.

**models/post.rb**

* has many votes (votes for/against this post)

```ruby
has_many :votes, as: :votable
```

Now for user, we need to define the association for two different attributes. First, the user will have many `ratings`, which refer to a regular 1:M association. This means that each user has many votes that they cast.

The second association will be polymorphic, and will refer to users voting for other users.

**models/user.rb**

* has many `ratings`
  * votes created by user
  * not polymorphic - regular one to many
* has many `votes`
  * votes for/against this user
  * polymorphic

```ruby
has_many :ratings, class_name: 'Vote'
has_many :votes , as: :votable
```

## Try it out

in terminal

```ruby
#list comments, posts, users
User.all
Vote.all
Post.all

#user up votes a post
User.first.ratings << Post.first.votes.create({value:1})

#user down votes a post
User.first.ratings << Post.last.votes.create({value:-1})

#user up votes a user
User.first.ratings << User.first.votes.create({value:1})

#list user ratings (votes cast by user)
User.first.ratings

#list user votes (votes about a user)
User.first.votes
```

### Resources

* [Rails guide - Polymorphic Models](http://guides.rubyonrails.org/association_basics.html#polymorphic-associations)
* [Rails Advanced Generators](http://railsguides.net/advanced-rails-model-generators/)