app.js

App.Router.map(function() {
  return this.resource("modelsA", function() {
    this.resource("modelA", { path: ":modelA_id" }, function() {
      return this.resource("modelsB", function() {
        this.resource("modelB", {
          path: ":modelB_id"
        });
        return this.route("new")
      });
    });
    this.route("new");
  });
});

App.ModelsBController = Ember.ObjectController.extend({
  needs: ['modelA'],
  save: function() {
    transaction = this.get('store').transaction();
    var modelB = transaction.createRecord(App.ModelB, {
      name: this.get('name'),
      modelA: ****CAN'T FIGURE OUT HOW TO GET REFERENCE TO THE MODEL A RECORD HERE.***
    });
    return transaction.commit();
  }
});

// Path to the new form for the Model B record is modelsA/[ID#]/modelsB/new

index.hbs

<script type="text/x-handlebars" id="modelA">
 {{#linkTo "modelsB.new"}}Add Model B{{/linkTo}}
</script>

<script type="text/x-handlebars"  id="modelsB/new">
{{view Ember.TextField valueBinding='name'}}
<button {{action "save"}} class='btn'>Create</button>
</script>