blocks|key|3691363|text|Rails有一个很好的+query+method，它使这个特定的查询更容易生成：|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|3691364|Comment.where(article_id:+article.id).select(:user_id).distinct.count|code-block|syntax|javascript|3691365|它生成以下SQL：|3691366|SELECT+DISTINCT+COUNT(DISTINCT+"comments"."user_id")+FROM+"comments"+WHERE+"comments"."article_id"+=+?|3691367|这种方法的好处是允许数据库完成繁重的工作。数据库通常比在Ruby中直接操作对象快几个数量级。|3691368|如果您愿意按以下方式修改您的Article类：|style|CODE|3691369|class+Article+<+ActiveRecord::Base
++has_many+:comments
++has_many+:commenters,+through:+:comments,+source:+:user
++has_many+:unique_commenters,+->+{+distinct+},+through:+:comments,+source:+:user
end|3691370|还可以使用以下代码生成查询：|3691371|article.unique_commenters|3691372|3691373|SELECT+DISTINCT+"users".*+FROM+"users"+INNER+JOIN+"comments"+ON+"users"."id"+=+"comments"."user_id"+WHERE+"comments"."article_id"+=+?|3691374|entityMap|0|LINK|mutability|MUTABLE|url|http://apidock.com/rails/ActiveRecord/QueryMethods/distinct^0|B|D|0|0|0|0|0|0|E|7|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|19|8|@]|9|@$A|1A|B|1B|1|1C]]|C|$]]|$1|D|3|E|5|F|7|1D|8|@]|9|@]|C|$G|H]]|$1|I|3|J|5|6|7|1E|8|@]|9|@]|C|$]]|$1|K|3|L|5|F|7|1F|8|@]|9|@]|C|$G|H]]|$1|M|3|N|5|6|7|1G|8|@]|9|@]|C|$]]|$1|O|3|P|5|6|7|1H|8|@$A|1I|B|1J|Q|R]]|9|@]|C|$]]|$1|S|3|T|5|F|7|1K|8|@]|9|@]|C|$G|H]]|$1|U|3|V|5|6|7|1L|8|@]|9|@]|C|$]]|$1|W|3|X|5|F|7|1M|8|@]|9|@]|C|$G|H]]|$1|Y|3|J|5|6|7|1N|8|@]|9|@]|C|$]]|$1|Z|3|10|5|F|7|1O|8|@]|9|@]|C|$G|H]]|$1|11|3|-4|5|6|7|1P|8|@]|9|@]|C|$]]]|12|$13|$5|14|15|16|C|$17|18]]]]

Rails has the wonderful <a href="http://apidock.com/rails/ActiveRecord/QueryMethods/distinct" rel="nofollow"><code>distinct</code> query method</a> which makes this particular query easy to generate:

<pre><code>Comment.where(article_id: article.id).select(:user_id).distinct.count
</code></pre>

which generates the following SQL:

<pre><code>SELECT DISTINCT COUNT(DISTINCT "comments"."user_id") FROM "comments" WHERE "comments"."article_id" = ?
</code></pre>

The benefit of this approach is you allow the database to do the heavy lifting. The database will often be several orders of magnitude faster than manipulating the objects directly in Ruby.

If you're willing to modify your <code>Article</code> class as follows:

<pre><code>class Article &lt; ActiveRecord::Base
 has_many :comments
 has_many :commenters, through: :comments, source: :user
 has_many :unique_commenters, -&gt; { distinct }, through: :comments, source: :user
end
</code></pre>

you can also use the following code to generate a query:

<pre><code>article.unique_commenters
</code></pre>

which generates the following SQL:

<pre><code>SELECT DISTINCT "users".* FROM "users" INNER JOIN "comments" ON "users"."id" = "comments"."user_id" WHERE "comments"."article_id" = ?
</code></pre>

blocks|key|3691299|text|尝试使用pluck和uniq|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|3691300|@article.comments.uniq.pluck(:user_id)|3691301|或者仅仅是pluck|3691302|@article.comments.pluck("DISTINCT+user_id")|3691303|entityMap|0|LINK|mutability|MUTABLE|url|http://api.rubyonrails.org/classes/ActiveRecord/Calculations.html#method-i-pluck|1|http://apidock.com/rails/ActiveRecord/QueryMethods/uniq^0|4|5|A|4|4|5|0|A|4|1|0|0|12|0|5|5|0|0|17|0^^$0|@$1|2|3|4|5|6|7|V|8|@$9|W|A|X|B|C]|$9|Y|A|Z|B|C]]|D|@$9|10|A|11|1|12]|$9|13|A|14|1|15]]|E|$]]|$1|F|3|G|5|6|7|16|8|@$9|17|A|18|B|C]]|D|@]|E|$]]|$1|H|3|I|5|6|7|19|8|@$9|1A|A|1B|B|C]]|D|@]|E|$]]|$1|J|3|K|5|6|7|1C|8|@$9|1D|A|1E|B|C]]|D|@]|E|$]]|$1|L|3|-4|5|6|7|1F|8|@]|D|@]|E|$]]]|M|$N|$5|O|P|Q|E|$R|S]]|T|$5|O|P|Q|E|$R|U]]]]

Try using <a href="http://api.rubyonrails.org/classes/ActiveRecord/Calculations.html#method-i-pluck" rel="nofollow"><code>pluck</code></a> and <a href="http://apidock.com/rails/ActiveRecord/QueryMethods/uniq" rel="nofollow"><code>uniq</code></a>

<code>@article.comments.uniq.pluck(:user_id)</code>

or just <code>pluck</code>

<code>@article.comments.pluck("DISTINCT user_id")</code>

blocks|key|866058|text|article.comments.collect(&:user).uniq{%7Cuser%7C+user.id}.count|type|code-block|depth|inlineStyleRanges|entityRanges|data|syntax|javascript|866059|unstyled|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|G|8|@]|9|@]|A|$B|C]]|$1|D|3|-4|5|E|7|H|8|@]|9|@]|A|$]]]|F|$]]

<pre><code>article.comments.collect(&amp;:user).uniq{|user| user.id}.count
</code></pre>

blocks|key|1361162|text|您可以使用Ruby的Set进行如下操作：|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1361163|unique_commenters+=+Set.new

comments.each+do+%7Ccomment%7C
++unique_commenters.add+comment.user
end

unique_commenters.to_a|code-block|syntax|javascript|1361164|您也可以只对集合执行commenters.uniq!操作，但我想这样做的效率较低。|1361165|entityMap^0|A|3|0|0|A|G|0^^$0|@$1|2|3|4|5|6|7|O|8|@$9|P|A|Q|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|R|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|S|8|@$9|T|A|U|B|C]]|D|@]|E|$]]|$1|M|3|-4|5|6|7|V|8|@]|D|@]|E|$]]]|N|$]]

you could use Ruby's <code>Set</code> for this:

<pre><code>unique_commenters = Set.new

comments.each do |comment|
 unique_commenters.add comment.user
end

unique_commenters.to_a
</code></pre>

You could also just do <code>commenters.uniq!</code> on the collection, but I guess that is less efficient.

blocks|key|3691337|text|我所知道的最好的方法是创建一个范围：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|3691338|class+Comment
++belongs_to+:user
++belongs_to+:article,+counter_cache:+true++

++scope+:count_by_user+->(user)+{+where(user_id:+user.id).count+}
end|code-block|syntax|javascript|3691339|然后：|3691340|@article.comments.count_by_user(user)+#+2|3691341|entityMap^0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|M|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|N|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|O|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|P|8|@]|9|@]|A|$E|F]]|$1|K|3|-4|5|6|7|Q|8|@]|9|@]|A|$]]]|L|$]]

The best way I know is create a scope:

<pre><code>class Comment
 belongs_to :user
 belongs_to :article, counter_cache: true 

 scope :count_by_user -&gt;(user) { where(user_id: user.id).count }
end
</code></pre>

Then:

<pre><code>@article.comments.count_by_user(user) # 2
</code></pre>

There are Articles and Comments. 

<pre><code>class Comment &lt; ActiveRecord::Base
 belongs_to :user
 belongs_to :article, counter_cache: true
end
</code></pre>

You can count number of comment by doing something like this:
@article.comments_count

The question is - how would you count the number of unique users who left a comment on a specific @article ? (note: a unique user can leave multiple comments)

here's a scenario:

<blockquote>
 <ol>
 <li>comment - user_id(1)</li>
 <li>comment - user_id(4)</li>
 <li>comment - user_id(1)</li>
 </ol>
 
 comments count = 3 
 commenters (unique user who left a comment) count = 2
</blockquote>

Count number of commenters

翻译质量差，导致语言生硬或混乱。

没有提供实际的解决方法或示例。

解答不清晰，无法理解或解决问题。

页面排版不美观，阅读体验差。

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有文章和评论。class Comment < ActiveRecord::Base belongs_to :user belongs_to :article, counter_cache: trueend您可以通过这样的操作来计算注释的数量：@article.comments_count问题是--你如何计算在一篇特定@文章上留下评论的唯一用户的数量？(注意:唯一的用户可以留下多个注释)下面是一

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