Mongoid在最后一次调用时返回第一个文档的错误? [英] The bug of mongoid returning first document when invoking last?

问题描述

我遇到了mongoid错误，当我调用方法last

I encountered a bug of mongoid, which return first document when I invoking the method last

class Post
  include Mongoid::Document
end

Post.create
Post.create
Post.first == Post.last #=> true

版本信息:

• 蒙古语:"5.0.2"
• mongodb:v3.2.1
• 导轨4.2.5

推荐答案

这不是Mongoid中的错误，而是您对first和last方法的期望中的错误.从精细版本5手册:

That's not a bug in Mongoid, that's a bug in your expectations of the first and last methods. From the fine version 5 manual:

#first⇒文档

注意:Mongoid以前在用户未明确提供排序参数的情况下添加了_id排序.这会导致性能问题，这是无法预料的，​​因此，如果未提供排序参数，则#first/#last将不再保证顺序.对于订单保证-必须明确提供一种排序.

Note: Mongoid previously added an _id sort when no sort parameters were provided explicitly by the user. This caused bad performance issues and was not expected, so #first/#last will no longer guarantee order if no sorting parameters are provided. For order guarantees - a sort must be explicitly provided.

为条件的选择器获取数据库中的第一个文档.

Get the first document in the database for the criteria's selector.

因此，first(从Mongoid5开始)相对于当前订单为您提供了查询中的第一个文档，但不再提供默认订单.您需要提供自己的 sort first像以前一样运行，请在查询中使用a>顺序.

So first (as of Mongoid5) gives you the first document in the query with respect to the current order but it no longer supplies a default order. You'll need to supply your own sort order in the query if you want first to behave like it used to.

类似于 last .

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