title: Eloquent work around for composite keys, creating new rows does not work but updating does
body: <p>I have 2 tables:</p>
  
  <pre><code>order_items
  - product_id
  - order_id
  - qty_ordered
  
  stock_movements
  - product_id
  - order_id
  - qty_picked
  </code></pre>
  
  <p>Order items has the following relationship to stock movements:</p>
  
  <pre><code>return $this-&gt;hasMany(StockMovement::class, 'product_id', 'product_id')
              -&gt;where('order_id', '=', $this-&gt;order_id);
  </code></pre>
  
  <p>This works fine for getting records and I can also increment existing rows like. </p>
  
  <pre><code>$this-&gt;stockMovement()-&gt;increment('picked', $int);
  </code></pre>
  
  <p>... however, using the create method, the order_id part of the relationship is missed out. I assume this is because its in the where statement and does not already exist. This does not work...</p>
  
  <pre><code>$this-&gt;stockMovement()-&gt;create(['picked' =&gt; $int]);
  </code></pre>
  
  <p>I realise that eloquent does not support composite keys but is there an elegant work around for this, short of building the query with the missing key manually. i.e.</p>
  
  <pre><code>$this-&gt;stockMovement()-&gt;create(['order_id' =&gt; $this-&gt;order_id, 'picked' =&gt; $int]);
  </code></pre>