title: add multi columns in many-to-many relation using fk-relation in xcrud
body: <p>I have three tables :</p>
  
  <p>orders :</p>
  
  <pre><code>id_order PK int
  id_client
  .
  .
  .
  etc ...
  </code></pre>
  
  <p>Products : </p>
  
  <pre><code>id_product PK int
  product_name  varchar(255)
  .
  .
  .
  etc ...
  </code></pre>
  
  <p>orders_products</p>
  
  <pre><code>id  PK int
  id_order  FK int
  id_product   FK int
  quantity    int
  discount    float
  </code></pre>
  
  <p>and Im using Xcrud as a crud framework based.</p>
  
  <p>this is my code :</p>
  
  <pre><code>    $xcrud = Xcrud::get_instance();
      $xcrud-&gt;table('orders');
      $xcrud-&gt;fk_relation('Products','id_order','orders_products','id_order','id_product','products', 'id_product','product_name');
  </code></pre>
  
  <ul>
  <li>So each order has multi products</li>
  <li>And each product has a quantity</li>
  </ul>
  
  <p>When I go to add an order it only shows me a multiselect field of products and it's inserting products find in the orders_products table.</p>
  
  <p>but I want to add for each product a quantity.</p>
  
  <p>How can I do that using xcrud ?</p>