title: why is nothing returned when using $match?
body: <p>Why is nothing returned when using <code>$match</code>?</p>
  <pre class="lang-js prettyprint-override"><code>{
    $lookup: {
      from: 'users',
      as: 'user',
      let: {
        &quot;blogIds&quot;: &quot;$blog.id&quot;
      },
      pipeline: [{
          $project: {
            id: 1,
            user_name: 1,
            picture: 1,
            blogs: 1
          },
        },
        {
          $match: {
            $expr: {
              $in: [&quot;$$blogIds&quot;, &quot;$blogs&quot;]
            }
          }
        }
      ]
    }
  }
  </code></pre>
  <p>If I remove <code>$match</code> and add <code>$addFields</code>, I get the following result:</p>
  <pre class="lang-js prettyprint-override"><code>{
    $lookup: {
      from: 'users',
      as: 'user',
      let: {
        &quot;blogIds&quot;: &quot;$blog.id&quot;
      },
      pipeline: [{
          $project: {
            id: 1,
            user_name: 1,
            picture: 1,
            blogs: 1
          },
        },
        {
          $addFields: {
            field: &quot;$$blogIds&quot;
          }
        },
      ]
    }
  }
  </code></pre>
  <p><a href="https://i.stack.imgur.com/gSdtH.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/gSdtH.jpg" alt="enter image description here" /></a></p>
  <p>Document example:</p>
  <p><a href="https://i.stack.imgur.com/KRaX8.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/KRaX8.png" alt="enter image description here" /></a></p>