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PORails join with max condition
 

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    <p>Given these two models</p> <pre><code>class Article &lt; AR::Base belongs_to :tag default_scope order('id DESC') end class Tag &lt; AR::Base has_many :articles scope :featured, order(:ordering).limit(5) end </code></pre> <p>I'm trying to join these two tables in a manner such I can retrieve a list of featured tags and for each of them the latest article in that tag with a single query, something like</p> <pre><code>Tag.featured.each do |tag| p tag.name p tag.articles.first.title # this will fetch the article in the tag with the highest id end </code></pre> <p>Written in this way this code has the <a href="http://guides.rubyonrails.org/active_record_querying.html#eager-loading-associations" rel="nofollow">(n+1) query problem</a>, I'm trying to optimize it because it will be run very frequently. I've excluded an <code>includes</code> call in the <code>featured</code> scope because it will load all the <code>Article</code> in the first 5 tags (a lot of articles...)</p> <p>In plain old SQL I can join this two tables using a query like this</p> <pre><code>SELECT tag_id, MAX(id) FROM articles GROUP BY tag_id # or in Rails Article.select('MAX(id), tag_id').group(:tag_id) +--------+---------+ | tag_id | MAX(id) | +--------+---------+ | 14 | 26787 | ... | 1 | 27854 | | 5 | 27780 | | 0 | 10953 | +--------+---------+ </code></pre> <p>as a join table I can retrieve all the data with a single query.</p> <p>How to port that in Rails and ActiveRecord?</p> <p><strong>update</strong></p> <p>The full query i need executed in AR context is</p> <pre><code>SELECT a.*, t.* FROM tags t JOIN (SELECT MAX(id) AS latest, tag_id FROM articles GROUP BY tag_id) j ON j.tag_id = t.id JOIN articles p ON j.latest = a.id LIMIT 5 </code></pre> <p>I tried <code>AR#find_by_sql</code> method with the latest query, I get the correct resultset but then I cannot navigate through objects</p> <pre><code>sql = '...' # This holds the previous query Tag.find_by_sql(sql).first.class =&gt; "Tag" Tag.find_by_sql(sql).first.articles.first =&gt; nil # why??? </code></pre> <p><strong>update 2</strong></p> <p>Tried also with </p> <pre><code>Tag.joins('JOIN ... all my query join part here ...').first.articles.first =&gt; nil </code></pre> <p>However I noticed that I can use my articles field directly as Tag fields, i.e. I can write</p> <pre><code>Tag.find_by_sql(...).first.title # where title is a field of Article class Tag.joins(...).first.title # where title is a field of Article class </code></pre> <p>but obviously I can not call <code>Article</code> instance methods.</p>
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    1. COI'm going to self-accept my own answer because of no other answers.
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