title: Mongo lookup join on objectid not working?
body: <p>For the collections:</p>
  
  <pre><code> data:
   { "_id" : ObjectId("57a"), "rep" : ObjectId("570"), "label" : "pat" }
   { "_id" : ObjectId("57b"), "rep" : ObjectId("571"), "label" : "pat" }
  
   rep:
   { "_id" : ObjectId("570") }
   { "_id" : ObjectId("571") }
  </code></pre>
  
  <p>query</p>
  
  <pre><code>db.rep.aggregate([{ $lookup: 
     {from: "data", localField:"rep", foreignField:"_id", as: "in_common" }
  }])
  </code></pre>
  
  <p>yields an empty set. </p>
  
  <p>The query should produce a result with two rows.</p>
  
  <p>How can I fix this?</p>