title: Rails Postgres many to many relationship with custom foreign key
body: <p>I have a fact table, <code>clients</code> with a bunch of businesses:</p>
  
  <pre><code>  bus_id, sales,  date  
      1,    $986,  1/1/2016  
      1,    $543,  1/2/2016  
      2,    $921,  1/1/2016  
      2,    $345,  1/2/2016
  </code></pre>
  
  <p>I want to create a table <code>opportunities</code> </p>
  
  <pre><code>  bus_id,  opportunity  
       1,     "Upsell"  
       1,    "Upsell More"
  </code></pre>
  
  <p>How do I create the <code>opportunities</code> table so that I could display the opportunities per <code>bus_id</code>?</p>