title: Adding remaining filed in $project after adding lookups in mongoose
body: <p>I'm doing a $lookup from an _id in Order schema, and its working as expected. But in <code>$project</code> how to add remaining keys. I have added my code below.</p>
  <p><strong>Product Collection:</strong></p>
  <pre><code>{
  &quot;_id&quot;: &quot;54759eb3c090d83494e2d804&quot;,
  &quot;product_name&quot;: &quot;sample product&quot;,
  &quot;image&quot;: &quot;default.png&quot;,
  &quot;price&quot;: 55,
  &quot;discount&quot;: 5,
  }
  </code></pre>
  <p><strong>Order list Collection</strong></p>
  <pre><code>   {
          &quot;user_name&quot;: &quot;sample1&quot;,
          &quot;product_list&quot;:[
          {
            &quot;product_id&quot;: &quot;54759eb3c090d83494e2d804&quot;
            &quot;quantity&quot;: 5
          }
          ]
      }
  </code></pre>
  <p><strong>lookups</strong></p>
  <pre><code>[
        {
          from: 'product',
          localField: 'product_list.product_id',
          foreignField: '_id',
          as: 'product_list.product_id',
          model: 'ProductModel',
        },
      ],
  </code></pre>
  <p><strong>$Project</strong></p>
  <pre><code>{
        user_name: true,
        product_list: {
          $map: {
            input: '$product_list.product_id',
            as: 'product',
            in: {
              product_name: '$$product.product_name',
            },
          },
        },
      }
  </code></pre>
  <p><strong>Current Result:</strong></p>
  <pre><code>{
      &quot;user_name&quot;: &quot;sample1&quot;,
      &quot;product_list&quot;:[
         &quot;product_id&quot;:{
            &quot;product_name&quot;: &quot;sample product&quot;
          }
       ]
  }
  </code></pre>
  <p>In this current result, the quantity field is missing. How to add in <code>$project</code>?. The expected result shown below</p>
  <p><strong>Expected Result:</strong></p>
  <pre><code>   {
        &quot;user_name&quot;: &quot;sample1&quot;,
         &quot;product_list&quot;:[
             {
                 &quot;product_id&quot;: {
                      &quot;product_name&quot;: &quot;sample product&quot;
                  }
                 &quot;quantity&quot;: 5
             }
           ]
     }
  </code></pre>