title: How to hide _id from Aggregation?
body: <p>I've this query:</p>
  
  <pre><code>produits = yield motor.Op(db.users.aggregate, [{"$unwind":"$pup"},{"$match":{"pup.spec.np":nomp}}, {"$group":{"_id":"$pup.spec.id","pup":{"$push":"$pup"}}}])
  </code></pre>
  
  <p>which gives me this result:</p>
  
  <pre><code>print produits
  
  {u'ok': 1.0, u'result': [{u'_id': None, u'pup': [{u'avt': {u'fto': ..all the results}}]}]}
  </code></pre>
  
  <p>so I can do:</p>
  
  <pre><code>prod = produits["result"]
  
  [{u'_id': None, u'pup': [{u'avt': {u'fto': ..all the results}}]}]
  </code></pre>
  
  <p>but how can I hide <code>"_id"</code> so that I can only get:</p>
  
  <pre><code>[{u'pup': [{u'avt': {u'fto': ..all the results}}]}]
  </code></pre>
  
  <p>in a normal query I would simply add something like <code>{"_id":0}</code> but here it doesn't work.</p>