title: MongoDB type update from NumberLong to String
body: <p>I've imported CSV file to my mongodb. CSV have separators as needed to mongo and was received from MySQL database with this query:<br>
  <code>SELECT * FROM csgo_users INTO OUTFILE 'b1.csv' FIELDS TERMINATED BY ',' ENCLOSED BY '"' LINES TERMINATED BY '\n';</code> <br>
  Also I've tried to receive CSV from Sequel Pro with it's export functions.
  <br>
  In my csv I have field with 17 characters length. In MySQL database this field has type VARCHAR but contains 17-digits number.<br>
  After importing csv to mongo I got this field with type <code>NumberLong</code>, but I want it to be <code>string</code>.<br>
  I've tried so far to:</p>
  
  <ul>
  <li>Change type in MySQL from <code>varchar</code> to <code>text</code>.</li>
  <li>Tried to import csv with additional flags <code>--headerline</code> and <code>--columnsHaveTypes</code></li>
  <li>Also I've tried to add separate fields without top line, with tag <code>--fields</code>.</li>
  <li><p>Tried commands as this: <br></p>
  
  <pre><code>db.csgo_users.find({"steam_id": {$type: 18}})
         .toArray()
         .map(function(v){
             v.steam_id = new String(v); 
             db.csgo_users.save(v)
          })
  </code></pre></li>
  </ul>
  
  <p>or this: <br></p>
  
  <pre><code>db.csgo_users.find({"steam_id": {$type: 18}})
    .toArray()
    .map(function(v){
       v.steam_id = new String(v.toSring());
       db.csgo_users.save(v)
    })
  </code></pre>
  
  <p><br>
   - I've tried a lot of solutions with <code>forEach()</code> like <a href="https://stackoverflow.com/a/29487693/6066986">this</a> or <a href="https://stackoverflow.com/a/4973632/6066986">this</a> or <a href="https://stackoverflow.com/a/7796204/6066986">this</a>
   etc.</p>
  
  <p><br>
  For the last one example of my tries I've got not string, but Object, <code>{"N",u","m","b","e","r","L","o","n","g"..}</code> but I want it to be "123456789", not Object. <br>
  I'm using MongoDB 3.4 <a href="https://docs.mongodb.com/manual/" rel="noreferrer">docs</a>.<br>
  <strong>So, my question is, how to change "field" type from <code>NumberLong</code> to <code>String</code>?</strong></p>