title: marshmallow flatten nested objects
body: <p>I have objects that look a little something like:</p>
  
  <pre><code>source = {
    id: 123,
    objects: [
      {
        random_data: aaaa,
        actual_data: {
          useful: 111,
          ...
        }
      }, 
      {
        random_data: bbbb,
        actual_data: {
          useful: 222,
          ...
        }
      }
    ]
  }
  </code></pre>
  
  <p>and while I could easily serialize that as is, it would be far more useful to transform it into the following format:</p>
  
  <pre><code>dest = {
    id: 123,
    objects: [
      {
        useful: 111,
        ...
      },
      {
        useful: 222,
        ...
      }
    ]
  }
  </code></pre>
  
  <p>Given I already have a schema for the innermost object (<code>actual_data</code> in the source above - it is a lot more complex than my example shows), what is the best way of flattening that nested object so I only get the objects from <code>actual_data</code> without the extra depth? I would have thought it would be something like</p>
  
  <pre><code>objects = Nested(UsefulSchema, many=True, load_from"&lt;something&gt;.useful_data")
  </code></pre>
  
  <p>but I have been unable to find a solution without preprocessing the data.</p>
  
  <p>NB I have simplified my example a lot to give the simplest form of my problem. In my actual code, the source data is a large list of similar objects from sqlalchemy.</p>