Mongoid：保存一个字符串数组会引发TypeError？

Question

我做错了什么？我有这个基于Mongoid的模型：

class ReadableThing
  include Mongoid::Document
  field :shop,    type: Integer
  field :user,                 type: Integer
  field :title,                type: String
  field :s_title,              type: String
  field :s_desc,               type: String
  field :is_part,              type: Boolean
  field :pieces,               type: Array

  def self.with_shop_only(shop, with_parts)
    ReadableThing.where(shop_id: shop, user: nil, is_part: with_parts)
  end

  def self.with_shop_and_user(shop, user_id, with_parts)
    ReadableThing.where(brokerage_shop_id: shop, user: user_id, is_part: with_parts)
  end
end

并且在运行此代码时：

ReadableThing.create({
  shop: 2,
  user: nil,
  title: "Ooohh samples",
  s_title: "SPL",
  s_desc: nil,
  is_part: true,
  pieces: ["this","is","a","sample"],
})

我收到此错误：

TypeError: no implicit conversion of String into Integer

我已经能够将这些东西追踪到“碎片”阵列，没有它，一切都很完美。但是，我无法弄清楚我做错了什么。

任何人都可以帮我一把吗？