ruby-on-rails – 用于ActiveModel :: Errors的未定义方法`model

class Exercise
  include Mongoid::Document
  field :name,:type => String
  field :description,:type => String

  belongs_to :group

  validates_presence_of :name,:description,:group
end

我有以下控制器：

class ExercisesController < ApplicationController
  respond_to :json

  def create
    @exercise = Exercise.create(params[:exercise])
    if @exercise.save
      respond_with @exercise
    else
      respond_with(@exercise.errors,:status => :unprocessable_entity)
    end
  end
end

模型在有效时保存正常但在运行以下行时：

respond_with(@exercise.errors,:status => :unprocessable_entity)

我收到以下错误

undefined method `model_name’ for ActiveModel::Errors:Class

填充了错误集合,因此我认为我的respond_with语法错误.