title: Convert to key-value in Mongodb
body: <p>Currently I have mongodb Database. Each document have field <code>data</code> which is key-value storage. 
  I want to convert it to </p>
  
  <pre><code>data:[
      {'k': key1, v: value1},
      {'k': key2, v: value2},
      {'k': key3, v: value3}]
  </code></pre>
  
  <p>My current strycture is: </p>
  
  <pre><code>data:{
      key1: value1,
      key2: value2,
      key3: value3}
  </code></pre>
  
  <p>So I can use search on keys (currently I'm using $exists but it's to slow and after converting I Could index data.k and use index for fast searching. 
  What is the fastest way to do so. Now I'm using pymongo individually edit every document individually and saving it. But it's to slow. Is there native way to do my job? </p>