How to Rescue from ActionDispatch::ParamsParser::ParseError in Rails 4

Turns out that further up the middleware stack, ActionDispatch::ShowExceptions can be configured with an exceptions app.

module Traphos  class Application < Rails::Application    # For the exceptions app    require "#{config.root}/lib/exceptions/public_exceptions"    config.exceptions_app = Traphos::PublicExceptions.new(Rails.public_path)  endend

Based heavily on the Rails provided one I am now using:

module Traphos  class PublicExceptions    attr_accessor :public_path    def initialize(public_path)      @public_path = public_path    end    def call(env)      exception    = env["action_dispatch.exception"]      status       = code_from_exception(env["PATH_INFO"][1..-1], exception)      request      = ActionDispatch::Request.new(env)      content_type = request.formats.first      body         = {:status => { :code => status, :exception => exception.class.name, :message => exception.message }}      render(status, content_type, body)    end    private    def render(status, content_type, body)      format = content_type && "to_#{content_type.to_sym}"      if format && body.respond_to?(format)        render_format(status, content_type, body.public_send(format))      else        render_html(status)      end    end    def render_format(status, content_type, body)      [status, {'Content-Type' => "#{content_type}; charset=#{ActionDispatch::Response.default_charset}",                'Content-Length' => body.bytesize.to_s}, [body]]    end    def render_html(status)      found = false      path = "#{public_path}/#{status}.#{I18n.locale}.html" if I18n.locale      path = "#{public_path}/#{status}.html" unless path && (found = File.exist?(path))      if found || File.exist?(path)        render_format(status, 'text/html', File.read(path))      else        [404, { "X-Cascade" => "pass" }, []]      end    end    def code_from_exception(status, exception)      case exception      when ActionDispatch::ParamsParser::ParseError        "422"      else        status      end    end  endend

To use it in a test environment requires setting config variables (otherwise you get the standard exception handling in development and test). So to test I have (edited to just have the key parts):

describe Photo, :type => :api do  context 'update' do    it 'attributes with non-parseable json' do       Rails.application.config.consider_all_requests_local = false      Rails.application.config.action_dispatch.show_exceptions = true      patch update_url, {:description => description}      response.status.should eql(422)      result = JSON.parse(response.body)      result['status']['exception'].should match(/ParseError/)      Rails.application.config.consider_all_requests_local = true      Rails.application.config.action_dispatch.show_exceptions = false    end  endend

Which performs as I need in a public API way and is adaptable for any other exceptions I may choose to customise.

This article (also from 2013) thoughtbot covers also this topic. They put their response inside this middleware service only if you requested json

if env['HTTP_ACCEPT'] =~ /application\/json/    error_output = "There was a problem in the JSON you submitted: #{error}"    return [      400, { "Content-Type" => "application/json" },      [ { status: 400, error: error_output }.to_json ]    ]else raise errorend