title: mongodb type change to array
body: <p>I have a field in mongodb that's a string. <code>{"field": "some text"}</code>, I want to convert them all into arrays. <code>{"field": ["some text"]}</code></p>
  
  <p>I know I can just loop through all the documents, get the field, then update, but I'm wondering if there's a cleaner way.</p>
  
  <p>Thanks.</p>