how to handle exceptions in JSON based RESTful code?

(I found the answer just before I hit [Post your question]. But this might help someone else as well...)

Use ActionController's rescue_from

The answer is to use ActionController's rescue_from, as described in this Guide and documented here. In particular, you can replace the default rendering of the default 404.html and 500.html files along these lines:

class ApplicationController < ActionController::Base  rescue_from ActiveRecord::RecordNotFound, :with => :record_not_foundprivate  def record_not_found(error)    render :json => {:error => error.message}, :status => :not_found  end end

If it helps anyone, this is what I did as a catch all for my purely json api:

In your ApplicationController that each specific controller inherits from, add

# app/controllers/api/v1/application_controller.rb# ...rescue_from StandardError do |exception|    render json: { :error => exception.message }, :status => 500end# ...

• based mostly off fearless_fool's answer

As a developer, you will also want to see traces (preferably with useful lines, filtering out gems). And make traces invisible for production:

  rescue_from StandardError do |exception|    # Handle only JSON requests    raise unless request.format.json?    err = {error: exception.message}    err[:backtrace] = exception.backtrace.select do |line|      # filter out non-significant lines:      %w(/gems/ /rubygems/ /lib/ruby/).all? do |litter|         not line.include?(litter)      end    end if Rails.env.development? and exception.is_a? Exception    # duplicate exception output to console:    STDERR.puts ['ERROR:', err[:error], '']                    .concat(err[:backtrace] || []).join "\n"    render :json => err, :status => 500  end