### Polymorphism in Rails
---
* Allows you to have a model **associated with more than one other model without the use of a join table**
* How?
    * Adding a **type** and **id** field to the table of the model you wish to make polymorphic
    * When creating the association using `rails g` and appending **name:references{polymorphic}**
    can be used to quickly establish the `_type` and `_id` polymorphic column names (all done by Rails).
---
#### Example: Person, Business & Interaction
* Interactive will belong to an Interaction which Person and Business will have many of:
```ruby
rails g model Person first_name last_name
rails g model Business name
rails g model Interaction description interactive:references{polymorphic}
# you can use any name for the polymorphic id and type, just as long as they make sense

rake db:migrate
```
* Add to Person and Business class (keep Interaction class the same)
```ruby
class Person < ApplicationRecord
   has_many :interactions, as: :interactive
end

class Business < ApplicationRecord
   has_many :interactions, as: :interactive
end

# This should defaulted by rails
class Interaction < ApplicationRecord
   belongs_to :interactive, polymorphic: true
end
```
* If you open `rails c` and then entered `Interaction.column_names`, you will see:
    * `interaction_id`: holds the **id** of the model that the interaction belongs to
    * `interaction_type`: holds the **type** of the model that the interaction belongs to
    * Both tell Rails which model this interaction should be associated with
* Adding the following controllers:
```ruby
# in terminal
rails g controller People index show new create edit update
rails g controller Businesses index show new create edit update
rails g controller Interactions new create edit update
```

* **Nest your resources** such that the polymorphic association is nested within each model!
* You will need some way to **parse** which object the polymorphic association belongs to:
    * Checking the **params** for the object's class and then **returning** the object in the db
```ruby
private
def context
   if params[:person_id]
      id = params[:person_id]
      Person.find(params[:person_id])
   else
      id = params[:business_id]
      Business.find(params[:business_id])
   end
end
```
* For your forms that use the polymorphic association:
```ruby
<%= form_for [obj1, obj2] do %>
   #...
<% end %>
```
    * `obj1` is the object returned from parsing the polymorphic association (ex. @context = context)
    * `obj2` is the object of the polymorphic class (ex. Interaction.new)