Demo

// Successful updating of a task
HTH.store.find(HTH.Project).get('firstObject').set('title','Updated the title!');

// Unsuccessful attempt to add a task to a project
var project = HTH.store.createRecord(HTH.Project, {title: "New" });
var task = HTH.store.createRecord(HTH.Task, {title: "New task" });
task.set('project',project);
project.get('tasks').pushObject(task);

Sproutcore

// Models

HTH.Project = SC.Record.extend({
  title: SC.Record.attr(String),
  tasks: SC.Record.toMany('HTH.Task',{
    inverse: 'project'
  })
});

HTH.Task = SC.Record.extend({
  title: SC.Record.attr(String),
  project: SC.Record.toOne('HTH.Project', {
    inverse: 'tasks'
  })
});

HTH.Project.FIXTURES = [ { guid: 1, title: 'Send man to moon', tasks: [1,2]}, 
                         { guid: 2, title: 'Master handstand', tasks: [3]}];
HTH.Task.FIXTURES    = [ { guid: 1, title: 'Find rocket',    project: 1}, 
                         { guid: 2, title: 'Find fuel',      project: 1},
                         { guid: 3, title: 'Stand on hands', project: 2}];

HTH.store = SC.Store.create().from(SC.Record.fixtures);

// Controller

HTH.projectsController = SC.ArrayProxy.create({
  content: HTH.store.find(HTH.Project)
});

Templates

  <script type="text/x-handlebars">
    {{#collection contentBinding="HTH.projectsController.content"}}
      <h3>{{content.title}}</h3>
      {{#collection contentBinding="content.tasks"}}
      <span>{{content.title}}</span>
      {{/collection}}
    {{/collection}}
  </script>