title: MongoDB query to make key-value with existing values
body: <p>My database has:</p>
  
  <pre><code>document1: {id: 'a', value: 1},
  document2: {id: 'b', value: 2},
  document3: {id: 'c', value: 3},
  </code></pre>
  
  <p>And I need get</p>
  
  <pre><code>{a: 1, b: 2, c: 3}
  </code></pre>
  
  <p>Could I do it using only mongo commands? </p>
  
  <p>Thanks!</p>