Rails: get validation key instead of i18n string

I am developing a Ruby on Rails 4.1.0 application that has a mobile client API and a backend for a different type of user. The API allows the mobile client to register a user, but I can't get the correct data, so in this case I need to tell the mobile client where the errors were. I am using application 3.2.4 for user management. So right now, if the mobile user wants to sign up and does not perform validations, this is the JSON I am returning to the mobile client:

{"errors": [
  "email": ["is not valid"]
]}

      

        
        
        
      

    

Since the mobile app will be multilingual, the app developers will ask me to return all errors with a key instead of a suggestion, so I have to return them something like:

{"errors": [
  "email": ["invalid"]
]}

      

        
        
        
      

    

But I cannot find a way to do this easily. When I run user.valid?

, the object user.errors

gets populated with i18n validation errors, so I can't get their keys.

I tried to create an i18n backend (inherit from I18n::Backend::Simple

and overwrite method translate

), but I'm having problems with parameters default

that I don't want to handle when sending confirmation emails, or resetting password instructions emails.

So, I want to know why my entry is invalid, but without the i18n line, only the key. Any idea on how to do this?

Thank!