title: How to pass a parameter in a Marshmallow schema
body: <p>Using Python, Flask and marshmallow, if I have a schema:</p>
  
  <pre><code>class ParentSchema(Schema):
      id = fields.Int(dump_only=True)
      children = fields.Nested('ChildSchema', dump_only=True)
  </code></pre>
  
  <p>and a class <code>Parent</code> which has a method:</p>
  
  <pre><code>class Parent():
      getChildren(self, params):
          pass
  </code></pre>
  
  <p>How do I get Marshmallow to pass the necessary parameters to <code>Parent.getChildren</code> when serialising the object and then populate <code>ParentSchema.children</code> with the results?</p>