ruby-on-rails – ruby??数组何时计数不等于数组中的元素数？

(rdb:1) @document.document_items
[]
(rdb:1) @document.document_items.count
2
(rdb:1) @document.document_items.length
0
(rdb:1) @document.document_items.size
0
(rdb:1) @document.document_items.class
Array
(rdb:1) @document
#<Document id: 1,title: "Recusandae quibusdam dicta deleniti et voluptate.",state: "published",site_id: nil,template_document_id: 2,most_important_message: "Quam totam corporis similique voluptatibus quaerat ...",delta: nil,approver_id: 1,author_id: 1,account_id: 1,updated_at: "2011-05-06 08:59:12",created_at: "2011-05-06 08:59:12">
(rdb:1) DocumentItem.find(:all)
[#<DocumentItem id: 1,title: "Et voluptatem officia voluptatem omnis voluptas.",body: "Nobis iste nostrum beatae corrupti ea qui debitis. ...",position: nil,document_id: 1,created_at: "2011-05-06 08:59:12",version: 1,is_heading: false,help_message: nil,optional: nil,template_question_id: nil>,#<DocumentItem id: 2,title: "Ipsum in odio laborum ut officia.",body: "Quas temporibus iusto quidem non repellat. Quia des...",template_question_id: nil>]

我的Document / DocumentItem模型的片段：

class Document < ActiveRecord::Base
    ...
    has_many :document_items
    ...
end

class DocumentItem < ActiveRecord::Base
    ...
    belongs_to :document
    ...
end

为什么document_items数组的数量与document_items中的元素数量不同？它是某种机械师的魔法吗？
(可能与：Ruby 1.92 in Rails 3: A Case where Array.length Does Not Equal Array.count?有关)

但是,所有这一切的问题是,为什么document_items是空的？连接已正确设置,因为这有效：

(rdb:1) DocumentItem.first.document
#<Document id: 1,created_at: "2011-05-06 08:59:12">