Mongoid:如何仅加载通过引用延迟加载的对象的某些字段? [英] Mongoid: How to load only some fields of an object you lazy load via reference?

问题描述

出于性能方面的考虑，在编写蒙古式查询时，我会尽可能多地使用only()关键字，以指定要加载的字段.

For performance reason, I use as often as possible the only() keyword when writing up a mongoid query in order to specify the fields I want to load.

通常的怀疑是，例如当我只希望用户的所有管理员电子邮件仅用于显示时.

The usual suspect, is for instance when I want a user's email of all my admins only for display purposes.

我会写:

User.where(:groups => :admins).only(:email).each do |u|
 puts u.email
end

之所以这样做，是因为我的用户模型中充满了很多数据，当我列出一堆电子邮件时，我很乐意忽略这些数据.

I do this because my user model are quite full of a lot of data that I can gladly ignore when listing a bunch of emails.

但是，现在让我们想象一下，我的用户是通过Project模型进行引用的，因此对于每个项目，我都可以这样做:project.user.多亏了mongoid的延迟加载，当我调用引用时，我的对象用户才会被实例化(并从数据库中查询).

However, now let imagine, that my users are referenced via a Project model, so that for each project I can do: project.user. Thanks to mongoid's lazy loading, my object user will only get instantiated (and queried from the DB) when I call upon the reference.

但是如果我想列出所有管理项目所有者的所有电子邮件怎么办?

But what if I want to list all the email of the owner of all admin project for instance ?

我会这样写:

Project.where(:admin_type => true).each do |p|
  puts p.user.email
end

这里的主要问题是这样做，我为每个项目加载了整个用户对象，并且如果有很多与查询匹配的项目可能变得很繁琐.那么如何只加载电子邮件?

The major problem here is that doing this, I load the entire user object for each projects, and if there are lots of project matching the query that could get pretty heavy. So how do I load only the emails ?

我可以这样做:

User.where(:_id => p.user_id).only(:email).first.email

但是，这显然破坏了简单地执行简单语法的目的:

But this obviously defeat the purpose of the nice syntax of simply doing:

p.user.email 

我希望我可以写类似的内容:p.user.only(:email).email，但是我不能.有什么想法吗?

I wish I could write something like: p.user.only(:email).email, but I can't. Any ideas ?

亚历克斯

推荐答案

Mongoid创建者的答案.还不可能它已作为功能请求添加.

Answer from creator of Mongoid. It's not possible yet. It's been added as feature request.

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