title: How can I artificially nest schemas in Marshmallow?
body: <p>In Marshmallow, is there a way to pass the current object to a <code>Nested</code> field in order to produce artificially nested serializations? For example, consider this object that I'm serializing:</p>
  
  <pre><code>example = Example(
      name="Foo",
      address="301 Elm Street",
      city="Kalamazoo",
      state="MI",
  )
  </code></pre>
  
  <p>I want to produce JSON for this that looks like this:</p>
  
  <pre><code>{
      "name": "Foo",
      "address": {
              "street": "301 Elm Street",
              "city": "Kalamazoo",
              "state": "MI"
      }
  }
  </code></pre>
  
  <p>Essentially, this would be a nested <code>AddressSchema</code> inside the <code>ExampleSchema</code>, something like this:</p>
  
  <pre><code>class AddressSchema:
      street = fields.String(attribute="address")
      city = fields.String()
      state = fields.String()
  
  class ExampleSchema:
      name = fields.String()
      address = fields.Nested(AddressSchema)
  </code></pre>
  
  <p>...but that doesn't quite do what I'd like. I can use a custom function, but I'd like to use a built-in method if possible.</p>