title: How to update without overwriting children?
body: <p>consider this existing data in a Firebase Realtime Database</p>
  
  <pre><code>db.ref('path/to/child').once('value')
  
  {
    "data":{
      "this":"value",
      "that":"value"
    }
  }
  </code></pre>
  
  <p>consider this set operation to the original data</p>
  
  <pre><code>db.ref('path/to/child').update({"data":{"this":"newValue"}})
  
  {
    "data":{
      "this":"newValue",
    }
  }
  </code></pre>
  
  <p>how do we perform an update so that the unchanged data is preserved</p>
  
  <pre><code>db.ref('path/to/child').foo({"data":{"this":"newValue"}})
  
  {
    "data":{
      "this":"newValue",
      "that":"value"
    }
  }
  </code></pre>