Mongoid-按参考文件查询 [英] Mongoid - querying by referenced document

问题描述

我有一个名为Ad的模型，它看起来像这样:

I have a model named Ad which looks like this:

class Ad
  include Mongoid::Document
  referenced_in :category
end

和类别模型:

class Category
  include Mongoid::Document
  referenced_in :domain
  references_many :ads
end

如何选择按域分类的广告? 我尝试使用Ad.where('category.domain_id' => domain.id)，但这不起作用.

How can I select Ads by domain? I have tried to use Ad.where('category.domain_id' => domain.id) but this does not work.

推荐答案

问题是MongoDB无法将Category记录映射到Ad记录.它所知道的就是Ad记录具有category_id字段，因此'category.domain_id'将始终不返回任何内容.查询中的点符号仅适用于嵌入式文档，不适用于引用(在MongoDB中仍是二等公民).

The problem is that MongoDB doesn't have any way of mapping a Category record to an Ad record. All it knows is that an Ad record has a category_id field so 'category.domain_id' will always return nothing. The dot notation inside queries works only for embedded documents, not references (which are still second-class citizens in MongoDB).

因此，要解决您的问题，您将需要2个查询:

So to solve your problem, you'll need 2 queries:

category_ids = Category.where(:domain_id => domain.id).map(&:_id)
Ad.where(:category_id.in => category_ids)

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