Custom failure app to render a custom json on auth failure

custom_auth_failure_app.rb

class CustomAuthFailure < Devise::FailureApp
  def respond
    self.status = 401 
    self.content_type = 'json'
    self.response_body = {"errors" => ["Invalid login credentials"]}.to_json
  end 
end

I've used this implementation and it works almost every time. The problem appears when I try to register a user with an existing email address. Rails will skip right to a PG exception. Any thoughts?

You're getting the PG exception probably because you're missing the validations on the User model.