How to Rescue from ActionDispatch::ParamsParser::ParseError in Rails 4 - ruby-on-rails

Question

Rails 4 adds an exception ActionDispatch::ParamsParser::ParseError exception but since its in the middleware stack it appears it can't be rescued in the normal controller environment. In a json API application I want respond with a standard error format.
This gist shows a strategy for inserting middleware to intercept and respond. Following this pattern I have:
application.rb:
module Traphos
class Application < Rails::Application
....
config.middleware.insert_before ActionDispatch::ParamsParser, "JSONParseError"
end
end
And the middleware is:
class JSONParseError
def initialize(app)
#app = app
end
def call(env)
begin
#app.call(env)
rescue ActionDispatch::ParamsParser::ParseError => e
[422, {}, ['Parse Error']]
end
end
end
If I run my test without the middleware I get (spec):
Failures:
1) Photo update attributes with non-parseable json
Failure/Error: patch update_url, {:description => description}, "CONTENT_TYPE" => content_type, "HTTP_ACCEPT" => accepts, "HTTP_AUTHORIZATION" => #auth
ActionDispatch::ParamsParser::ParseError:
399: unexpected token at 'description=Test+New+Description]'
Which is exactly what I would expect (ParseError that I can't rescue_from).
Now with the only change to add in the middleware above:
2) Photo update attributes with non-parseable json
Failure/Error: response.status.should eql(422)
expected: 422
got: 200
And the log shows that the standard controller action is being executed and returning a normal response (although since it didn't receive any parameters it didn't update anything).
My questions:
How can rescue from ParseError and return a custom response. Feels like I'm on the right track but not quite there.
I can't work out why, when the exception is raised and rescued, that the controller action still proceeds.
Help much appreciated, --Kip

Answer 1

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

Answer 2

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 error
end