How to rescue ActionDispatch::ParamsParser::ParseError and return custom API error in rails 5?

Question

Whenever sending malformed JSON against my API-only Rails 5.x application, I get an exception and Rails is returning the entire stack trace as JSON. Obviously I'd like to respond with a nicely, custom, formatted error.

=> Booting Puma
=> Rails 5.0.0.1 application starting in development on http://localhost:3000
=> Run `rails server -h` for more startup options
Puma starting in single mode...
* Version 3.6.0 (ruby 2.3.0-p0), codename: Sleepy Sunday Serenity
* Min threads: 5, max threads: 5
* Environment: development
* Listening on tcp://localhost:3000
Use Ctrl-C to stop
Started POST "/api/v1/identities/" for ::1 at 2016-10-26 18:42:32 +0200
  ActiveRecord::SchemaMigration Load (0.3ms)  SELECT "schema_migrations".* FROM "schema_migrations"
Error occurred while parsing request parameters.
Contents:

{
    "whatever": "ewewgewewtwe"
    "malformed_json": ";
}

ActionDispatch::ParamsParser::ParseError (822: unexpected token at '{
    "whatever": "ewewgewewtwe"
    "malformed_json": ";
}'):

actionpack (5.0.0.1) lib/action_dispatch/http/parameters.rb:71:in `rescue in parse_formatted_parameters'
actionpack (5.0.0.1) lib/action_dispatch/http/parameters.rb:65:in `parse_formatted_parameters'
actionpack (5.0.0.1) lib/action_dispatch/http/request.rb:366:in `block in POST'
rack (2.0.1) lib/rack/request.rb:57:in `fetch'
rack (2.0.1) lib/rack/request.rb:57:in `fetch_header'
actionpack (5.0.0.1) lib/action_dispatch/http/request.rb:365:in `POST'
actionpack (5.0.0.1) lib/action_controller/metal/params_wrapper.rb:282:in `_wrapper_enabled?'
actionpack (5.0.0.1) lib/action_controller/metal/params_wrapper.rb:231:in `process_action'
activerecord (5.0.0.1) lib/active_record/railties/controller_runtime.rb:18:in `process_action'
actionpack (5.0.0.1) lib/abstract_controller/base.rb:126:in `process'
actionpack (5.0.0.1) lib/action_controller/metal.rb:190:in `dispatch'
actionpack (5.0.0.1) lib/action_controller/metal.rb:262:in `dispatch'
actionpack (5.0.0.1) lib/action_dispatch/routing/route_set.rb:50:in `dispatch'
actionpack (5.0.0.1) lib/action_dispatch/routing/route_set.rb:32:in `serve'
actionpack (5.0.0.1) lib/action_dispatch/journey/router.rb:39:in `block in serve'
actionpack (5.0.0.1) lib/action_dispatch/journey/router.rb:26:in `each'
actionpack (5.0.0.1) lib/action_dispatch/journey/router.rb:26:in `serve'
actionpack (5.0.0.1) lib/action_dispatch/routing/route_set.rb:725:in `call'
rack (2.0.1) lib/rack/etag.rb:25:in `call'
rack (2.0.1) lib/rack/conditional_get.rb:38:in `call'
rack (2.0.1) lib/rack/head.rb:12:in `call'
activerecord (5.0.0.1) lib/active_record/migration.rb:552:in `call'
actionpack (5.0.0.1) lib/action_dispatch/middleware/callbacks.rb:38:in `block in call'
activesupport (5.0.0.1) lib/active_support/callbacks.rb:97:in `__run_callbacks__'
activesupport (5.0.0.1) lib/active_support/callbacks.rb:750:in `_run_call_callbacks'
activesupport (5.0.0.1) lib/active_support/callbacks.rb:90:in `run_callbacks'
actionpack (5.0.0.1) lib/action_dispatch/middleware/callbacks.rb:36:in `call'
actionpack (5.0.0.1) lib/action_dispatch/middleware/executor.rb:12:in `call'
actionpack (5.0.0.1) lib/action_dispatch/middleware/remote_ip.rb:79:in `call'
actionpack (5.0.0.1) lib/action_dispatch/middleware/debug_exceptions.rb:49:in `call'
actionpack (5.0.0.1) lib/action_dispatch/middleware/show_exceptions.rb:31:in `call'
railties (5.0.0.1) lib/rails/rack/logger.rb:36:in `call_app'
railties (5.0.0.1) lib/rails/rack/logger.rb:24:in `block in call'
activesupport (5.0.0.1) lib/active_support/tagged_logging.rb:70:in `block in tagged'
activesupport (5.0.0.1) lib/active_support/tagged_logging.rb:26:in `tagged'
activesupport (5.0.0.1) lib/active_support/tagged_logging.rb:70:in `tagged'
railties (5.0.0.1) lib/rails/rack/logger.rb:24:in `call'
actionpack (5.0.0.1) lib/action_dispatch/middleware/request_id.rb:24:in `call'
rack (2.0.1) lib/rack/runtime.rb:22:in `call'
activesupport (5.0.0.1) lib/active_support/cache/strategy/local_cache_middleware.rb:28:in `call'
actionpack (5.0.0.1) lib/action_dispatch/middleware/executor.rb:12:in `call'
actionpack (5.0.0.1) lib/action_dispatch/middleware/static.rb:136:in `call'
rack (2.0.1) lib/rack/sendfile.rb:111:in `call'
railties (5.0.0.1) lib/rails/engine.rb:522:in `call'
puma (3.6.0) lib/puma/configuration.rb:225:in `call'
puma (3.6.0) lib/puma/server.rb:578:in `handle_request'
puma (3.6.0) lib/puma/server.rb:415:in `process_client'
puma (3.6.0) lib/puma/server.rb:275:in `block in run'
puma (3.6.0) lib/puma/thread_pool.rb:116:in `block in spawn_thread'

Previously I think it was possible to add a middleware and handle the exception as following:

# in config/application.rb

config.middleware.insert_before(ActionDispatch::ParamsError,'BadRequestError')

# middleware

class BadRequestError
    def initialize(app)
        @app = app
    end

    def call(env)
        begin
            @app.call(env)
        rescue ActionDispatch::ParamsParser::ParseError
            Api::ApiController.action(:raise_bad_request).call(env)
        end
    end
end

However it seems that the middleware ActionDispatch::ParamsError was removed from Rails 5 ;

I also tried with other middlewares (e.g. ActionDispatch::ShowExceptions), rescuing different errors, but my raise_bad_request action is somehow never called.

Am I missing something, doing something wrong, or is there another way of doing that with Rails 5?

Thanks!

Answer 1

Updated the answer to work with Rails 5.1 and newer. (Thanks @Edwin Meyer) config.middlware.use needs a class now, instead of a string.

Also I mention now, that I've put the middleware under app/middleware. So no need to require.

---

I had the same problem, and it was very simple to solve in my case.

Just use in application.rb:

config.middleware.use CatchJsonParseErrors

# Instead of
# config.middleware.insert_before ActionDispatch::ParamsParser, "CatchJsonParseErrors"
# I used in my Rails 4 app

My middleware looks like this (from Catching Invalid JSON Parse Errors with Rack Middleware):

# app/middleware/catch_json_parse_errors.rb

class CatchJsonParseErrors
  def initialize(app)
    @app = app
  end

  def call(env)
    begin
      @app.call(env)
    rescue ActionDispatch::ParamsParser::ParseError => error
      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
    end
  end
end

Then when I send an invalid json the response looks like this:

{"status":400,"error":"There was a problem in the JSON you submitted: 743: unexpected token at '{ \"foo\": \"bar\" '"}

It is necessary that you provide the correct ACCEPT header.

Hope it helps you too.