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Routing Concerns allow you to declare common routes that can be reused inside others resources and routes.

concern :commentable do
  resources :comments
end

concern :image_attachable do
  resources :images, only: :index
end

These concerns are used in Resources routing:

resources :messages, concerns: [:commentable, :image_attachable]

or in a scope or namespace:

namespace :posts do
  concerns :commentable
end
Methods
C
Instance Public methods
concern(name, callable = nil, &block)

Define a routing concern using a name.

Concerns may be defined inline, using a block, or handled by another object, by passing that object as the second parameter.

The concern object, if supplied, should respond to call, which will receive two parameters:

* The current mapper
* A hash of options which the concern object may use

Options may also be used by concerns defined in a block by accepting a block parameter. So, using a block, you might do something as simple as limit the actions available on certain resources, passing standard resource options through the concern:

concern :commentable do |options|
  resources :comments, options
end

resources :posts, concerns: :commentable
resources :archived_posts do
  # Don't allow comments on archived posts
  concerns :commentable, only: [:index, :show]
end

Or, using a callable object, you might implement something more specific to your application, which would be out of place in your routes file.

# purchasable.rb
class Purchasable
  def initialize(defaults = {})
    @defaults = defaults
  end

  def call(mapper, options = {})
    options = @defaults.merge(options)
    mapper.resources :purchases
    mapper.resources :receipts
    mapper.resources :returns if options[:returnable]
  end
end

# routes.rb
concern :purchasable, Purchasable.new(returnable: true)

resources :toys, concerns: :purchasable
resources :electronics, concerns: :purchasable
resources :pets do
  concerns :purchasable, returnable: false
end

Any routing helpers can be used inside a concern. If using a callable, they're accessible from the Mapper that's passed to call.

# File actionpack/lib/action_dispatch/routing/mapper.rb, line 1859
def concern(name, callable = nil, &block)
  callable ||= lambda { |mapper, options| mapper.instance_exec(options, &block) }
  @concerns[name] = callable
end
concerns(*args)

Use the named concerns

resources :posts do
  concerns :commentable
end

concerns also work in any routes helper that you want to use:

namespace :posts do
  concerns :commentable
end
# File actionpack/lib/action_dispatch/routing/mapper.rb, line 1875
def concerns(*args)
  options = args.extract_options!
  args.flatten.each do |name|
    if concern = @concerns[name]
      concern.call(self, options)
    else
      raise ArgumentError, "No concern named #{name} was found!"
    end
  end
end