Ember: Update ObjectController property from ArrayController action?

Disclaimer: I'm quite new to Ember. Very open to any advice anyone may have.

I have a action in a ArrayController that should set an ObjectController property upon completion. How I can access the right context to set that property when creating a new Object?

Here is abbreviated app code show my most recent attempt:

ChatApp.ConversationsController = Ember.ArrayController.extend({
  itemController: 'conversation',
  actions: {
    openChat: function(user_id, profile_id){
      if(this.existingChat(profile_id)){
        new_chat = this.findBy('profile_id', profile_id).get('firstObject');
      }else{
        new_chat = this.store.createRecord('conversation', {
          profile_id: profile_id,
        });
        new_chat.save();
      }
      var flashTargets = this.filterBy('profile_id', profile_id);
      flashTargets.setEach('isFlashed', true);
    }
  },
  existingChat: function(profile_id){
    return this.filterBy('profile_id', profile_id).get('length') > 0;
  }
});

ChatApp.ConversationController =  Ember.ObjectController.extend({
  isFlashed: false
});

The relevant template code:

{{#each conversation in controller itemController="conversation"}}
  <li {{bind-attr class="conversation.isFlashed:flashed "}}>
    <h3>Profile: {{conversation.profile}} Conversation: {{conversation.id}}</h3>
    other stuff
  </li>
{{/each}}