title: Why does mongoexport query with $exist fail?
body: <p>I am trying to mongoexport (Version 2.6) MongoDB data into csv format using the command as follows</p>
  
  <pre><code>mongoexport --port 27017 -d test -q "{userId:{$exists:true} , name:'John'}"-c user_datas -f userId --csv -o /myOutFile.csv
  </code></pre>
  
  <p>and i got this error message:</p>
  
  <pre><code>assertion: 16619 code FailedToParse: FailedToParse: First character in field must be [A-Za-z$_]: offset:9 of:{userId:{true},name:John}
  </code></pre>
  
  <p>according to error message that something happened on '$exists' that caused the error . 
  whats wrong with my command?</p>