title: Filling in with documents with default values after find/aggregate
body: <p>I have a collection:</p>
  
  <pre><code>{ "name" : "A", "value" : 1, "date" : ISODate("2014-01-01T00:00:00.000Z") }
  { "name" : "B", "value" : 7, "date" : ISODate("2014-01-01T00:00:00.000Z") }
  { "name" : "A", "value" : 3, "date" : ISODate("2014-01-02T00:00:00.000Z") }
  { "name" : "B", "value" : 8, "date" : ISODate("2014-01-02T00:00:00.000Z") }
  { "name" : "B", "value" : 8, "date" : ISODate("2014-01-03T00:00:00.000Z") }
  { "name" : "A", "value" : 5, "date" : ISODate("2014-01-04T00:00:00.000Z") }
  { "name" : "A", "value" : 4, "date" : ISODate("2014-01-05T00:00:00.000Z") }
  </code></pre>
  
  <p>The document for A on 3rd Jan 2014 is not available. When I do a find/aggregate on A, I would like the document to appear in my result set with a default value (or better, value to be same as previous date). For example:</p>
  
  <pre><code>{ "name" : "A", "value" : 1, "date" : ISODate("2014-01-01T00:00:00.000Z") }
  { "name" : "A", "value" : 3, "date" : ISODate("2014-01-02T00:00:00.000Z") }
  { "name" : "A", "value" : 3 (or default value -1), "date" : ISODate("2014-01-03T00:00:00.000Z") }
  { "name" : "A", "value" : 5, "date" : ISODate("2014-01-04T00:00:00.000Z") }
  { "name" : "A", "value" : 4, "date" : ISODate("2014-01-05T00:00:00.000Z") }
  </code></pre>
  
  <p>How can this be done?</p>