render :polymorphic

"Hey! Get that excess conditional logic out of that view!"

render :polymorphic is a simple plugin which adds a :polymorphic parameter to the ActionView::Base#render method. This adds an opinionated method of rendering a partial at a derived path. Useful in situations where you have views that belong to polymorphic resources.

Show Rather Than Tell:

Examples assume these associations:

class Message < ActiveRecord::Base
  belongs_to :messageable, :polymorphic => true
end

class User < ActiveRecord::Base
  has_many :messages, :as => :messageable
end

class Group < ActiveRecord::Base
  has_many :messages, :as => :messageable
end

before render :polymorphic

# in app/views/messages/index.html.erb
<%- if message.messageable.is_a?(User) %>
  # lot's of user-specific view code here
<%- elsif messageable.messageable.is_a?(Group) %>
  # lot's of group-specific view code here
<%- end %>

Personally, I despise huge blocks of conditional logic wrapping lots of view code. The above could be written a bit more nicely without using render :polymorphic as such:

# in app/views/messages/index.html.erb
<%- if message.messageable.is_a?(User) %>
  <%= render :partial => 'user_index', :locals => { :message => message } %>
<%- elsif messageable.messageable.is_a?(Group) %>
  <%= render :partial => 'group_index', :locals => { :message => message } %>
<%- end %>

But there's an abstraction to be made there, which is precisely what render :polymorphic does:

# in app/views/messages/index.html.erb
<%= render :polymorphic => message.messageable, :locals => { :message => message } %>

Which will in turn render a partial at app/views/users/messages_index.html.erb (assuming message.messageable is a User). And there we go, a neatly encapsulated pattern for getting excess conditional logic out of the view.