title: convert foreign field objectid to string in mongodb
body: <p>Here I'm joining two collection with using $lookup. please help me to fix this</p>
  <pre><code>Product.aggregate([
     {
        $lookup: {
            from: &quot;product_categories&quot;,
            localField: &quot;category_id&quot;,
            foreignField: &quot;_id&quot;, &lt;== i'm unable access this
            as: &quot;category&quot;
        }
     }
  ])
  </code></pre>