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copied!<p>Let's preface this with it's not the answer you are looking for...</p> <p>Now, with that out of the way, here is what I tried and how it relates to the two links I posted in the comments of the question.</p> <pre><code>class User < ActiveRecord::Base # self-referential association - more on this later belongs_to :myself, class_name: "User", foreign_key: :id scope :w_computed_metric, ->() { select{[id, (id *2).as(computed_metric)]} } scope :w_age_in_seconds, ->() { select{[id, (extract('epoch from now()') - extract('epoch from users.created_at')).as(age_in_seconds)]} } scope :w_default_attributes, ->() { select{`*`} } def self.compound_metric scope = User.w_default_attributes.select{(b.age_in_seconds / a.computed_metric).as(compound_metric)} scope = scope.joins{"inner join (" + User.w_computed_metric.to_sql + ") as a on a.id = users.id"} scope = scope.joins{"inner join (" + User.w_age_in_seconds.to_sql + ") as b on b.id = users.id"} end sifter :sift_computed_metric do (id * 2).as(computed_metric) end sifter :sift_age_in_seconds do (extract(`epoch from now()`) - extract(`epoch from users.created_at`)).as(age_in_seconds) end def self.using_sifters_in_select User.w_default_attributes.joins{myself}.select{[(myself.sift :sift_computed_metric), (myself.sift :sift_age_in_seconds)]} end def self.using_from scope = User.w_default_attributes scope = scope.select{[(age_in_seconds / computed_metric).as(compound_metric)]} scope = scope.from{User.w_computed_metric.w_age_in_seconds} end end </code></pre> <p>So, running <code>User.compound_metric</code> in console will yield the results you are looking for - a <code>User</code> object with the additional attributes: <code>computed_metric</code>, <code>age_in_seconds</code>, and <code>compound_metric</code>. Unfortunately, this violates the third constraint you placed on an acceptable answer. Oh well...</p> <p>I also tried a few other things (as you can see from above):</p> <p>First point of note is the self-referential association, which I'm quite proud of - even though it doesn't get us to where we want to go.</p> <pre><code>belongs_to :myself, class_name: "User", foreign_key: :id </code></pre> <p>This nifty piece of code lets you access the same object through a join. Why is this important? Well, Squeel will only allow you to access associations through the <code>joins{}</code> method unless you pass it a string of SQL. This lets us use the <code>sifter</code>s feature of Squeel - in this case not to filter the results, rather to include <em>aggregate</em> columns from the db and let Squeel do the heavy lifting of aliasing and joining the statements. You can test it with the</p> <pre><code>def self.using_sifters_in_select User.w_default_attributes.joins{myself}.select{[(myself.sift :sift_computed_metric), (myself.sift :sift_age_in_seconds)]} end </code></pre> <p>The beauty of sifters to acheive this is the chainability and syntatic sugar - it is very flat and readable.</p> <p>The last bit I tried playing with is <code>.from{}</code>. Before this question, I didn't even know it existed. I was soooo excited with the possibility that I had missed something so simple as including a source for a query (in this case a sub-select). Testing with using_from</p> <pre><code>def self.using_from scope = User.w_default_attributes scope = scope.select{[(age_in_seconds / computed_metric).as(compound_metric)]} scope = scope.from{User.w_computed_metric.w_age_in_seconds} end </code></pre> <p>results in a TypeError:</p> <pre><code>TypeError: Cannot visit Arel::SelectManager from /home/prg10itd/projects/arel/lib/arel/visitors/visitor.rb:28:in `rescue in visit' from /home/prg10itd/projects/arel/lib/arel/visitors/visitor.rb:19:in `visit' from /home/prg10itd/projects/arel/lib/arel/visitors/to_sql.rb:348:in `visit_Arel_Nodes_JoinSource' from /home/prg10itd/projects/arel/lib/arel/visitors/visitor.rb:21:in `visit' from /home/prg10itd/projects/arel/lib/arel/visitors/to_sql.rb:139:in `visit_Arel_Nodes_SelectCore' from /home/prg10itd/projects/arel/lib/arel/visitors/to_sql.rb:121:in `block in visit_Arel_Nodes_SelectStatement' from /home/prg10itd/projects/arel/lib/arel/visitors/to_sql.rb:121:in `map' from /home/prg10itd/projects/arel/lib/arel/visitors/to_sql.rb:121:in `visit_Arel_Nodes_SelectStatement' from /home/prg10itd/projects/arel/lib/arel/visitors/visitor.rb:21:in `visit' from /home/prg10itd/projects/arel/lib/arel/visitors/visitor.rb:5:in `accept' from /home/prg10itd/projects/arel/lib/arel/visitors/to_sql.rb:19:in `accept' </code></pre> <p>(and yes, I'm testing against a local copy of the Arel and Squeel). I'm not familiar enough with the internal workings of Arel to resolve the issue without further effort (and most likely a fork of Arel). It does appear that Squeel just passes the <code>from{}</code> method to the Arel <code>from()</code> method without doing anything (beyond the rest of the magic that is Squeel).</p> <p>So where does that leave us? A solution that works, but is not as nice and elegant as I wish it was - but maybe someone else can leverage this to a better solution.</p> <p>PS - this is with Rails v3.2.13 and the respective version of Arel. The source for Rails v4 and Arel are quite different and not tested for this.</p>

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