Post.js

export default Relay.createContainer(Post, {
  initialVariables: {
    post_id: null,
    id: btoa('Post:' + 1),
  },

  prepareVariables: prevVariables => {
    return {
      ...prevVariables,
      id: btoa('Post:' + prevVariables.post_id),
      post_id: parseInt(prevVariables.post_id, 10)
    }
  },

  fragments: {
    node: () => Relay.QL`
      fragment on Post {
        post_id
        title
        body
        created_at
      }
    `
  }
});