Provide a generic interface for requires: complex type.

http://pastebin.com/QQV0NJ5K

  # Entity
  module API
  module Entities
    class Article < Grape::Entity
      expose :title, documentation: { type: 'string', desc: 'Title' }
      expose :body, documentation: { type: 'string', desc: 'Body' }
    end
  end
end

  # API
  desc "Create an article"
  params do
    requires :article, type: API::Entities::Article, documentation: { example: "aklsdfj" }
  end
  post '/articles' do
    puts params
    article = Article.create(params(:title, :body))
    represent(article, env)
  end

Currently:

# curl -X POST -H "Content-Type:application/json" 0:9292/v1/articles -d '{"title":"hello","body":"world"}'  
# {"error":"article is missing"}
# curl -X POST -H "Content-Type:application/json" 0:9292/v1/articles -d '{"article":{"title":"hello","body":"world"}}'
# {"error":"article is invalid"}

Start by reproducing the two request cases shown for the requires :article declaration and compare their missing and invalid responses. Trace the requires entry point and the post '/articles' example; done means a complex entity type can be accepted and validated through the generic interface without the current errors.