:constraints => {:format => /json/}, :defaults => { :format => 'json' }

JSON = {:constraints => {:format => /json/}, :defaults => { :format => 'json' }}

Example::Application.routes.draw do
  namespace :foobar do
    resources :apples, JSON
    resources :bananas, JSON
    # ...
  end
end

def json_resources(name)
  resources name, :constraints => {:format => /json/}, :defaults => { :format => 'json' }
end

Example::Application.routes.draw do
  namespace :foobar do
    json_resources :apples
    json_resources :bananas
    # ...
  end
end

# ...

Example::Application.routes.draw do
  namespace :foobar do
    [:apples, :bananas, ...].each{|n|json_resources(n)}
  end
end

def json_resources(*args)
  default_options = { :constraints => {:format => /json/}, :defaults => {:format => 'json'} }
  options = args.extract_options!
  args << default_options.deep_merge(options)
  resources args do
    yield if block_given?
  end
end

Example::Application.routes.draw do
  namespace :foobar do
    json_resources :apples
    json_resources :bananas
    # ...
  end
end

...actionpack-3.2.6/lib/action_dispatch/routing/route_set.rb:363:
in `add_route': Invalid route name: 'foobar_[:apples {:constraints=>
{:format=>/json/}, :defaults=>{:format=>"json"}}]_index' (ArgumentError)

json_resources :apples do
  get 'a_custom_action', :on => :member
end

def json_resources(name)
  resources name, :constraints => {:format => /json/}, :defaults => { :format => 'json' }
end

Example::Application.routes.draw do
  namespace :foobar do
    resources :apples, :constraints => {:format => /json/}, :defaults => { :format => 'json' } do
      get 'a_custom_action', :on => :member
    end
    json_resources :bananas
    # ...
  end
end