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

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

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)

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

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

Try it out

in terminal

#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
• Rails Advanced Generators