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    copied!<p>Similar answer to <a href="https://stackoverflow.com/a/11872798/624590">pdevisser's</a>: You could use something similar to the answer given on this question: <a href="https://stackoverflow.com/a/307724/624590">https://stackoverflow.com/a/307724/624590</a></p> <p>Which would basically result in:</p> <pre><code>class User has_many :selling_transactions, :class_name =&gt; 'Transaction', :foreign_key =&gt; 'seller_id' has_many :buying_transactions, :class_name =&gt; 'Transaction', :foreign_key =&gt; 'buyer_id' has_many :incomplete_sales, :class_name =&gt; 'Transaction', :foreign_key =&gt; 'seller_id', :conditions =&gt; { :buyer_id =&gt; nil } has_many :incomplete_purchases, :class_name =&gt; 'Transaction', :foreign_key =&gt; 'buyer_id', :conditions =&gt; { :seller_id =&gt; nil } def incomplete_transactions incomplete_sales + incomplete_purchases end end </code></pre> <p><strong>EDIT</strong>: Alright, not totally sure how to set it up the way you wish (using has_many), but something along these lines might work for you:</p> <pre><code>class User has_many :selling_transactions, :class_name =&gt; 'Transaction', :foreign_key =&gt; 'seller_id' has_many :buying_transactions, :class_name =&gt; 'Transaction', :foreign_key =&gt; 'buyer_id' def incomplete_transactions Transaction.where("(buyer_id = ? and seller_id = NULL) or (seller_id = ? and buyer_id = NULL)", id, id) end end </code></pre> <p>The where statement will return an <code>ActiveRecord::Association</code>, so you can follow it with limit (or other activerecord functions) when calling it.</p>
 

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