Client-Side IDs with Ember Data

Why Client-Generated IDs?

In most client-side frameworks (e.g. Backbone, Angular, Ember) models created in the web browser get temporary IDs (client IDs) and get their permanent ID from the server after a successful save.

For client-heavy applications, generating IDs on the client can simplify interaction with the server. Consider the case where the user creates a post with several attachments. You’ll need to make sure that the post is saved and has its ID resolved before attempting to save the child models in order to preserve this relationship. Take a look at this Ember issue and you’ll start to appreciate the complexity of this use case.

It can be liberating to know the ID of a model before it is saved. While uploading an attachment, you could associate it with other models or send someone a link to where the attachment will be. Users could work offline and save records to the database later without worrying about how to resolve and update IDs.

Because we’re using UUIDs, you could hypothetically find any record in your application given only an ID. I’ve not seen this used for anything practical yet, but there could be interesting possibilities if we kept a global store of IDs and their associated objects on the client.

If you find yourself generating secret IDs for resources (think account activation URLs) you may save yourself some code by using unique IDs. Security-wise, it really can’t hurt.

A Brief History of (My Experience With) Client-Side IDs

I first encountered client-generated IDs when working with Spine. Looking back at older versions you can see that Spine included a UUID generator. Every model was was given a unique ID upon creation.

Spine Author, Alex Maccaw, was critical of Backbone’s temporary client IDs:

… a solution that Backbone uses is generating an internal cid (or client id). You can use this cid temporarily before the server responds with the real identifier… I’m not such a fan of that solution, so I’ve taken a different tack with Spine.

However, in one commit that says it all (“add CIDs (inspired by Backbone)”) Alex brought Spine inline with Backbone’s server-generated ID strategy.

Despite this new direction, Spine remained flexible and is a great platform for client-generated IDs. Backbone isn’t as friendly for client-generated IDs, mainly because it uses the existence of a permanent ID to determine whether a record has been persisted. But, as Zed Shaw would say, Programming Motherf*er.

class App.Model extends Backbone.Model
  initialize: ->
    unless @id
      @set('id', utils.guid()) unless @id
      @_new = true
      @once 'sync', => @_new = false

  isNew: ->

Client-Side IDs with Ember Data and PostgreSQL

Ember Data is rarin' to go with client-generated IDs. Override the dedicated method in the RESTAdapter and you’re golden.

  generateIdForRecord: ->
    'xxxxxxxx-xxxx-4xxx-yxxx-xxxxxxxxxxxx'.replace /[xy]/g, (c) ->
      r = Math.random()*16|0
      v = if c == 'x' then r else (r&0x3|0x8)

That code and the inclusion of a specification for client-generated IDs on gives me confidence that Ember will continue to be a good fit for this approach.

Even though we’re talking about client-generated IDs, there will be times when the server needs to generate IDs of its own. If you’re using Mongo, you’re in luck – it already uses unique IDs. Here’s how you can whip up some matching unique IDs in PostgreSQL and Rails 4 with a couple of migrations.

# Enable UUID generation in PostgreSQL
class SetupDatabase < ActiveRecord::Migration
  def change
    enable_extension "uuid-ossp"
class CreateDocuments < ActiveRecord::Migration
  def change
    # Use {id: :uuid} to use UUIDs as the primary key.
    create_table :areas, id: :uuid do |t|
      t.string :name
      # t.belongs_to :category will use an integer. You'll need to setup your
      # associations like this.
      t.uuid :category_id


I haven’t hit any major pain points with client-generated IDs yet but here are some quick considerations before you dive in:

  • Model.first and Model.last won’t behave the way you expect in Rails, since the IDs may not be sequential.
  • There are some rumblings on the Internet about the impact of unique IDs relation join performance, but I couldn’t find anything conclusive.
  • URLs are uglier.

So give client-generated IDs a try if you’ve got a good use case. It can go a long way to making a user experience less dependent on a round trip to the server.

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