In Rails, has_many and belongs_to associations are pretty clear and simple. A User has_many contacts. A Contact belongs_to a user. But what if you want something that could belongs to either a User or a Contact, but not both?

For instance, you could have comments about users or contacts. A given comment could belong_to a User, or it could belong_to a Contact. Because it can't be about both a User and a Contact, you can't have separate foreign keys for each. But because the names of the classes are different, you can't set up a standard association using a single foreign key.

The answer is to use polymorphic associations. Rails makes it easy to solve this problem by creating an abstract interface to represent something that can be commented on. In this case, it's representing either a User or a Contact.

Inside the comments table in the database, you simply need

1. The id of this thing that can be commented on (the foreign key).
2. Another column which specifies which specific type of thing you're talking about in this case.

We can call this abstract interface commentable, and use the following syntax to specify the relationships:

class Comment < ActiveRecord::Base
  belongs_to :commentable, polymorphic: true
end
 
class Contact < ActiveRecord::Base
  has_many :comments, as: :commentable
end
 
class User < ActiveRecord::Base
  has_many :comments, as: :commentable
end