Grails: Saving nested objects using XML data binding.

Question

I'm creating a REST service in Grails to accept data from a python script. The python script generates an XML representation of the object graph and submits it to the controller. Things work great for my flat objects, but I can't figure out how to handle the case where a domain object contains a Set of children objects. For unrelated reasons, my DOA layer is pure Java JPA.

For example, my domain classes (Leaving out getters/setters/etc):

class Schedule {
  String name;
  @OneToMany;
  HashSet<Step> steps;
}

class Step {
    String name;
    @ManyToOne;
    Schedule schedule;
}

My python script generates XML like the following:

<schedule>
  <name>Foo</name>
  <steps>
     <step>
       <name>Bar</name>
     </step>
     <step>
        <name>Blatz</name>
     </step>
  </steps>
</schedule>

In my controller I have this:

def save = {
   def schedInstance = new Schedule(params['schedule'])
   ...
}

The steps property never gets populated. If I dump params out to a log the steps data is all jammed together (In my example above it would yield steps: "BarBlatz"

I have to be doing something terribly wrong. I would imagine that this is a common task. Everything I've been able to find about nesting objects is related to command objects. I don't want to have to duplicate my domain object code in a command object if I can avoid it.