validation_example.rb

class MyController
  validate :username  => String,
           :permalink => String,
           :user      => { 
                           :age      => Fixnum,
                           :numbers  => Array
                         }
           :on => :update
  
  def update
    # this will never fail, as it was validated before.
    params[:user][:numbers].last
  end
end

Consider:

class BananaController
  def create
    validate_args banana_id: String,
                        banana_colour: String,
                        banana_owner: {
                          username: 'pcalcado',
                          city: 'pcalcado'
                        }
  end

  def edit
    validate_args banana_id: String,
                        new_banana_colour: String,
                        banana_owner: {
                          username: 'pcalcado',
                          city: 'pcalcado'
                        }
  end
end

Instead:

class BananaController
  def create
    validate_args banana_id: String,
                        banana_colour: String,
                        banana_owner: BananaOwnerParams
  end

  def edit
    validate_args banana_id: String,
                        new_banana_colour: String,
                        banana_owner: BananaOwnerParams
  end
end

Have a look at https://github.com/solnic/virtus (for coercions)