foo.js

// embedded
{
  post: {
    title: "whatever",
    comments: [
      {
        id: 1,
        comment: "something"
      }
    ]
  }
}

// sideloaded

{
  post: {
    title: "whatever",
    comment_ids: [1]
  },
  comments: [
    {
      id: 1,
      comment: "something"
    }
  ]
}