title: MongoDB - find result where value is null or less than X
body: <p>I have a collection in MongoDB with two dates that define whether or not something is current. So I have an "end_date" that can be null, or may have a time value. An item that current has a null end_date or a time in the future. My query looks like this: </p>
  
  <pre><code>program_enrollments.find( {"start_date":{"$lte":1376982000},"end_date":[null,{"$gte ":1376982000}],"client":"52002d02cc94a31a0f000000"}, [] )
  </code></pre>
  
  <p>This looks proper to me, but do I need a different approach? I don't want to have a boolean flag that says whether or not the dates are current if I can avoid it. </p>