title: Using `includes` in Rails ActiveRecord by condition, without foreign key
body: <p>I have a model called Score, and a model called Adjustment. There is no association between them in the database, but they both have a field named <code>course</code>, and a field named <code>date</code>. An Adjustment belongs to a Score if they have the same <code>course</code> and the same <code>date</code>. So we can have multiple Adjustments belonging to the same Score.</p>
  
  <p>I want to be able to get all Scores from the database along with their Adjustments, something along the lines of:</p>
  
  <p><code>scores = Score.all.includes(adjustments)</code></p>
  
  <p>So that, for each score, I should be able to do something like</p>
  
  <p><code>score.adjustments</code></p>
  
  <p>and get all adjustments that belong to that score.</p>
  
  <p>I know I can do this using n+1 queries, but I'd like a more optimal solution. </p>