title: Mongoid querys memory limit
body: <p>I have around 1 million records in my database. 
  When I do:</p>
  
  <pre><code>MyModel.distinct(:my_field) 
  </code></pre>
  
  <p>getting this error -</p>
  
  <pre><code>failed with error 17217: "exception: distinct too big, 16mb cap"
  </code></pre>
  
  <p>Tried to do it via aggregation - getting the same error.</p>
  
  <pre><code>group = {"$group" =&gt; {"_id" =&gt; {"my_field" =&gt; "$my_field"}}}
  MyModel.collection.aggregate(group)
  
  failed with error 16389: "exception: aggregation result exceeds maximum document size (16MB)"
  </code></pre>
  
  <p>Is there any way in Mongoid to increase memory limit from 16mb to 100mb? Or is any other ways of getting those distinct results?
  Or how to manage large queries with Mongoid? </p>