title: AWS DocDB $project Requires Outputs, doesnt match mongo API
body: <p>I am using the $project stage in an aggregation pipeline using Pymongo on AWS DocumentDB, like so:</p>
  <pre><code>{ '$project': { 'foo': 0 } }
  </code></pre>
  <p>This results in the following error:</p>
  <blockquote>
  <p>$projection requires at least one output field</p>
  </blockquote>
  <p>As far as I can tell, the mongo docs state that as of Mongo version 3.4 you can use $project to specify <em>exclusions</em> of fields (see: <a href="https://www.mongodb.com/docs/v4.0/reference/operator/aggregation/project/" rel="nofollow noreferrer">https://www.mongodb.com/docs/v4.0/reference/operator/aggregation/project/</a>). The example they provide looks like:</p>
  <pre><code>db.books.aggregate( [ { $project : { &quot;lastModified&quot;: 0 } } ] )
  </code></pre>
  <p>Does docdb not match this part of the API spec? or am I missing something?</p>
  <p>EDIT: I'm also unable to use the _id filter specifically, which is also highlighted in mongo docs.</p>
  <pre><code>{'$project': {'_id': 0}}
  </code></pre>