title: how to pass dynamic value to $in in mongodb aggregation
body: <p>I have two collections 1 is campaigns and other is orders.
  I have to filter orders for each campaign. So what I am doing is that I'm fetching all the campaigns and after that I'm looking up for the orders that matches some specific criteria.</p>
  <pre><code>[
       { 
         $match: { type: 'FOLLOWUP' } 
       },
       {
         $lookup: {
           from: 'orders',
           as: 'orders',
           pipeline: [
             {
                $match: {
                   'status': { $in: '$activeFilter' }
                }
             }
           ]
         }
       }
  ]
  </code></pre>
  <p>In above example <code>status</code> contain some specific string and the <code>activeFilter</code> has array of string containing active status for that campaign. <code>activeFilter</code> is an array but I'm getting error that <code>$in</code> needs an array.</p>
  <p>Any help would be appreciated.</p>