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POParent entity sorted by votes difference
 

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    <p>I have three models that look like this (I just left the stuff important for the question):</p> <pre><code>class Symbol &lt; ActiveRecord::Base has_many :mnemonic end class Mnemonic &lt; ActiveRecord::Base belongs_to :symbol has_many :mnemonic_votes end class MnemonicVote &lt; ActiveRecord::Base belongs_to :mnemonic attr_accessible :vote_up end </code></pre> <p>:vote_up is of boolean type which if true means someone upvoted the mnemonic, and if false means someone downvoted it.</p> <p>I would like to get top three mnemonics by vote difference. Let's say there are 5 mnemonic records in the database with the following number of up/down votes (MnemonicVote records with true/false as :vote_up field):</p> <pre><code>mnemonic up down total mnemonic1 3 2 1 mnemonic2 17 3 14 mnemonic3 2 5 -3 mnemonic4 11 7 4 mnemonic5 5 5 0 </code></pre> <p>I would like to get the following three mnemonics (with counts) by descending order:</p> <pre><code>mnemonic2 14 mnemonic4 4 mnemonic1 1 </code></pre> <p>I wrote this actual code which gives me the result I want, but I am aware it sucks and I don't like how I did it because the data gets grouped and sorted <em>after</em> all the MnemonicVote records associated with a certaing Mnemonic record are fetched from the DB:</p> <pre><code>@mnemonics = Mnemonic.where(symbol_id: self.id) # here I fetch all Mnemonics associated with Symbol table @mnemonics.sort_by { |mnemonic| mnemonic.votes_total }.reverse! return @mnemonics.take(3) </code></pre> <p>where mnemonic.votes_total is a calculated attribute on Mnemonic object. I would like to get the same result by using a single AR (or even SQL) query. How can this be accomplished? Thanks.</p>
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