title: mongodb query using _id object in lookup localField
body: <p>we are trying to match _id object to mId foreignfield which is not working.
  looking for mondodb docs. they do not have anything on this. Is it possible with mongodb query or not? </p>
  
  <p>_id as object in document</p>
  
  <pre><code>   "_id" : ObjectId("56ab6663d69d2d1100c074db"),
  </code></pre>
  
  <p>mId as String in document</p>
  
  <pre><code>"mId" : "56ab6663d69d2d1100c074db",
  </code></pre>
  
  <p>query as below:</p>
  
  <pre><code> collection.aggregate([
                  {
                    $lookup:
                      {
                        from: "category",
                        localField: "_id",
                        foreignField: "mId",
                        as: "categories"
                      }
                 }
              ])
  </code></pre>
  
  <p><strong>UDPATE</strong></p>
  
  <p>as a summary, mongodb does not support type coercion in $lookup. so to need above working i have to have _id and mId as <strong>ObjectId</strong> type in documents itself to make it work.</p>