title: Find documents with array of objectIds
body: <p>I have an array of objectIds represented as strings.</p>
  
  <pre><code>["594c7a30f8f774268a2cdd14","594c7a30f8f774268a2cdd13"]
  </code></pre>
  
  <p>In my aggregation, I dynamically build a match object with this array along with other fields.  </p>
  
  <pre><code>match["_id"] = { $in: classes};
  </code></pre>
  
  <p>This doesn't work and I assume becuase it's just an array of strings instead of objectIDs like ObjectId("594c7a30f8f774268a2cdd14").  </p>
  
  <p>How do I build the match so that _id search works with the array?</p>