How to set the default value of Ember.Select whose contents are from the server side by ember.js data?

Question

I am a ember.js novice and trying to rewrite the CRUD pages of a project in ember.js. But I meet a problem about Ember.Select.

Steps:

1. On the create page, there is a DropDown control, whose content is fetched from the server side.
2. After a user clicks submmit button, action "save" is invoked. I add a breakpoint in "save" action to watch distributiontarget.get('serviceDataModel').
3. If I do not click the Dropdown box, the value of distributiontarget.get('serviceDataModel') is undefined. If I clicked the dropdown control and select one, the value of distributiontarget.get('serviceDataModel') is the value of url.

I think the reason is that the default value is not set for the Ember.Select after the data are returned from server. But I could not find a way to set the default vlaue. Ember Select set default value introduced a method. But this.store.find('distribution') returen a promise. How could we get the first object? In additon, how could I select an item in DropDown when I am going to work on the edit page?

APP.DistributiontargetsCreateController= Ember.ObjectController.extend({
    distributions: function() {
        return this.store.find('distribution');
    }.property(),
    actions: {
        save: function(distributiontarget) {
            ..............
            debugger;
        }
    }
});

APP.Distribution = DS.Model.extend({
    name: attr(),
    url: attr()
});

APP.Distributiontarget = DS.Model.extend({
    organizationId : attr('string'),
    name : attr('string'),
    deliveryType : attr('string'),
    host : attr('string'),
    baseDir : attr('string'),
    username : attr('string'),
    password : attr('string'),
    serviceDataModel : attr('string')
});


<form>
   <div class="control-group required">
       <label class="control-label">Username</label>
       <div class="controls">
           {{ view Ember.TextField valueBinding="username" }}
        </div>
    </div>
             ..................
    <div class="control-group required">
        <label class="control-label">Default Data Model</label>
        <div class="controls">
        <span>
            {{view Ember.Select
                    content=distributions
                    optionValuePath="content.url"
                    optionLabelPath="content.name"
                    valueBinding="serviceDataModel"
            }}
        </span>
        </div>
    </div>
    <div class="form-actions">
        <button type="submit" {{action save this}}> Save Distribution</button>
    </div>
</form>

The data returned from the server:

{
  "distributions" : [ {
    "name" : "Test One",
    "url" : "http://192.168.204.164",
    "id" : "1"
  }, {
    "name" : "Test Two",
    "url" : "http://192.168.204.166",
    "id" : "2"
  }
 }