title: Select one of each type from collection
body: <p>I have a Meteor collection that has a field called <code>type</code>. I'm wondering if there's a way to select one document of each <code>type</code>. For example, if I had the following documents in my collection (sorted by descending <code>createdAt</code>)</p>
  
  <pre><code>Document #1: {type = "apple", createdAt: ...}
  Document #2: {type = "apple", createdAt: ...}
  Document #3: {type = "grape", createdAt: ...}
  Document #4: {type = "orange", createdAt: ...}
  Document #5: {type = "orange", createdAt: ...}
  Document #6: {type = "grape", createdAt: ...}
  Document #7: {type = "apple", createdAt: ...}
  ...
  </code></pre>
  
  <p>Then how could I select 3 documents each with a unique type? In this example, I would need Document #1, #3, and #4.</p>