Double definition required in belongsTo and hasMany with FIXTURES?

I'm testing out a very simple model relationship. I'm using the Fixture adapter before I switch to a true back-end. Somewhat following from here and here, I've found I need to define both sides of the relationship in each Fixture.

Is that a restriction of the Fixture adapter or will the backend/database also have to store both sides (which is non-standard for something like SQL)?

Models:

// app/models/comment.js
...
export default DS.Model.extend({
  text: DS.attr('string'),
  author: DS.attr('string'),
  //post: DS.belongsTo('post', {async: true})  // async only needed on one side - true??
  post: DS.belongsTo('post')
}).reopenClass({
FIXTURES: [
    {
      id: 1,
      text: 'Text of the comment goes right in here',
      author: 'Fred',
      post: 1
    },
    {
      id: 2,
      text: 'TWO 2222 comment two here',
      author: 'Barney',
      post: 1
    },
    {
      id: 3,
      text: 'third comment here',
      author: 'Wilma',
      post: 2
    },
...

// app/models/post.js
...
export default DS.Model.extend({
  text: DS.attr('string'),
  comments: DS.hasMany('comment', {async: true})
}).reopenClass({
    FIXTURES: [
    {
      id: 1,
      text: 'Post ONE - Lorem ipsum dolor sit amet, consectetur risus.',
      comments: [1,2]  // Why is this required??
    },
    {
      id: 2,
      text: 'Post TWO - consectetur adipiscing elit. Sed gravida faucibus risus.',
      comments: [3,4]  // Why is this required?
    },
...

My ember info:

• Ember Inspector 1.8.2
• Ember 1.12.0
• Ember Data 1.0.0-beta.18
• jQuery 1.11.3