title: how use mongoexport to export only specific fields in a sub-document
body: <p>is there a way to export only specified fields in a sub-document while using mongoexport? mongo docs says to just use -f field1, field2 etc... but that only works with top level fields. i have a document inside of the main document that also has fields. is there any way to get only those?</p>
  
  <p>Example:</p>
  
  <pre><code>{
      "topField1": "topValue1",
      "topField2": "topValue2",
      "subDoc1: {
                    "subField1": "subValue1",
                    "subField2": "subValue2"
                }
  }
  </code></pre>
  
  <p>is there a way to specify that i ONLY get the field subField2?</p>
  
  <p>i know in a regular mongo query i could use "subDoc1.subField2" which would simply 
  <code>return {"$oid": 122432432, {"subDoc1":{"subField2": "subValue2"}}</code> but this doesn't seem to work with <code>mongoexport</code>.</p>
  
  <p>also i want to export as <code>json</code>.</p>