The problem with server-rendered errors

By: Brian Cardarella

The 3rd most popular question with ember-validations is how can the library work with server-rendered validation errors, such as the ones returned with ember-data.

The short answer for now: it can’t.

(btw, 2nd most popular question is about uniqueness/remote validations and the 1st most popular question is when will I provide a dist/ directory… I’ll cover the 2nd Q in an upcoming blog post. As far as dist/ its never going to happen, ever)

Here is the problem. When you are dealing with a client-side model and a server-rendered model there won’t always be a 1-to-1 representation of the model. In those cases you can rely on ember-data’s serializer to transform the properties on a server-rendered error object to ones that exist on the client data model. How about properties that don’t exist at all in any form on the client? You could have a validation error on something only meant for server-rendered purposes. How do we best handle this?

Let’s imagine for a moment that we can properly map all the properties back to their client-side equivalents. Now what? How do you resolve these validation errors? How do you know in the UI when the validation error has been resolved to clear the error message? Are you preventing data submission until your client model is valid? If the errors are happening server-side the odds are high that these are not validations that can be known to be resolved on the client unless you do another data submission and wait to see how the server responds.

So to re-cap the two issues are:

  1. Potential lack of context on which properties errors can map back to
  2. Inability to know when server-rendered validation errors are satisfied on the client

To start to consider a possible solution I think we need to step back and consider the ultimate goal of client side validations. In my mind this is purpose: to help the user submit valid data to the server.

Client side validations are just UI sugar. They are there to guide your users. ember-validations only has model-layer concerns, which means you have to provide how the validation errors are displayed on your UI. This is why I also wrote ember-easyForm which handles the complexity of what I consider to be best practices of how validation messages should be displayed and cleared. To fix this problem would have to tackle it from both sides:

  1. How will server-rendered errors be stored in the validation graph? (ember-validations)
  2. How will server-rendered errors be displayed and resolved in the client? (ember-easyForm)

Storing server-rendered errors

If you are already using ember-data then your data model is handling this for you already. IMO you should never mix your validations into your data model, they should be mixed into your controller:

var UserController =
Ember.ObjectController.extend(Ember.Validations.Mixin, {
  validations: {
    firstName: {
      presence: true

This way the controller has its own errors object which will not clash with the errors object on your data model. One possibility of referencing the model’s server-rendered errors is to have a base validator that is not part of the validation graph but who’s errors can be used for presentation purposes.

Displaying server-rendered errors

So how do you properly display these error messages? Do you try to associate them with a property? What if that property is not represented by a form input? What if you aren’t even using a form? How do you know when to clear the errors?

I believe this is a complex issue. My first pass at handling this in EasyForm will be to display all of the server-rendered errors in a single place. An upcoming version of EasyForm will simply group all errors in base and display them. These errors will not clear out due to any corrections made by the client. They will only clear when some other action clears out those errors, for example when ember-data itself clears out or changes the content of its errors object.


This is far from ideal. This moves us away from the “best practices” for high conversion forms outlined by Luke Wroblewski. But it is better than not guiding your users. If the server errors for any given reason we don’t want our users sitting there without any feedback.

I am very interested in other approaches and brainstorming on the best direction for this. Please feel free to comment below.