Ember data saving a relationship

I'm having difficult saving a one-to-many relationship in ember data. I have a relationship like this:

App.ParameterSet = DS.Model
    name: DS.attr("string")
    regions: DS.hasMany("App.Region")

App.Region = DS.Model
    name: DS.attr("string")

If I were to do something like this:

parameterSet = App.ParameterSet.find(5)
@transaction = @get("store").transaction()
@transaction.add(parameterSet)
region1 = App.Region.find(10)
region2 = App.Region.find(11)
parameterSet.set("name", "foo")
parameterSet.get("regions").pushObject(region)
@transaction.commit()

Then I would like to see a PUT request with payload like this:

api/ParameterSets/5

{parameterSet: {name: "foo", regionIds:[10, 11]}}

but instead I get this:

{parameterSet: {name: "foo"}}

I don't care about the relationship back from child to parent but if I add parameterSet: DS.belongsTo("App.ParameterSet") to the App.Region model then I get 2 PUT requests to the regions url for the two new relationships which is not really what I want.

I guess this is a many-to-many relationship really which I'm not sure is yet supported but any ideas on how to achieve what I've described? Thanks