如何通过“计数"找到记录?使用rails和mongoid的关联? [英] How can I find records by "count" of association using rails and mongoid?

问题描述

使用这些模型:

class Week
  has_many :proofs
end
class Proof
  belongs_to :week
end

我想做类似的事情:

Week.where(:proof.count.gt => 0)

仅查找具有多个证明的星期.

To find only weeks that have multiple proofs.

似乎有一个答案可以解决这个问题:

There is one answer that seems to address this:

但是在本示例中，由于ID与证明一起存储，因此在Week中不存在诸如proof_ids之类的属性.例如，这不起作用:

But in this example, there is no such attribute as proof_ids in Week since the ids are stored with the proofs. This does not work for example:

Week.where(:proof_ids.gt => 0)

如何进行此查询?从概念上讲很简单，但我不知道如何使用mongo或mongoid做到这一点.

How is this query possible? Conceptually simple but I can't figure out how to do this with mongo or mongoid.

类似地，我想按证明的数量进行排序，例如:

Similarly, I'd like to order by the number of proofs for example like:

Week.desc(:proofs.size)

但这也不起作用.

我确实意识到计数器缓存是我两个特定问题的一种选择，但我也希望能够进行查询.

I do realize that a counter-cache is an option to both my specific questions but I'd also like to be able to do the query.

在此先感谢您的帮助.

推荐答案

有了Rails(并且没有counter_cache)，您可以这样做:

With rails (and without counter_cache), you could do:

class Week < ActiveRecord::Base
  has_many :proofs

  def self.by_proofs_size
    sort_by { |week| week.proofs.size }
  end

  def self.with_at_least_n_proofs(n = 1)
    select { |week| week.proofs.size >= n }
  end
end

尽管这些操作中的每一个都会产生2个查询，但这还是不理想的.

Even though each of those operations produces 2 queries, this is far from ideal.

使用范围(错误?)重复该对查询(每个操作==> 4个查询):

The pair of queries is repeated (=> 4 queries for each operation) with scopes (bug?):

scope :with_at_least_n_proofs, -> (n = 1) { select { |w| w.proofs.size >= n } }
scope :by_proofs_size, -> { sort_by { |w| w.proofs.size } }

理想的情况是使用counter_cache

The ideal is probably to use counter_cache

scope :with_at_least_n_proofs, -> (n = 1) { where('proofs_count >= ?', n) }
scope :by_proofs_size, -> { order(proofs_count: :desc) }

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