title: group objects array underscore js
body: <p>I'm getting an array back from an URL which looks like this:</p>
  
  <pre><code>[
     {
       id : 1,
       product_id : 101,
       price_new : 80,
       price_old : 90
     },
     {
       id : 2,
       product_id : 101,
       price_new : 80,
       price_old : 90
     },
     {
       id : 3,
       product_id : 102,
       price_new : 80,
       price_old : 90
     },
  ]
  </code></pre>
  
  <p>I would like to transform this, to:</p>
  
  <pre><code>[
     {
       product_id : 101,
       offers : [
       {
         id: 1
         price_new : 80,
         price_old : 90
       },{
         id: 2
         price_new : 80,
         price_old : 90
       }]
     },
     {
       product_id : 102,
       offers: [{
         id : 3,
         price_new : 80,
         price_old : 90
       }]
     },
  ]
  </code></pre>
  
  <p>Anyone who knows how to get this done using underscore js?
  Would love to get a solution using underscore cause we're using it in our entire project and it looks more clean, so...</p>