Rails: Validating JSON request format before API controller methods

Question

I've built a rails API that receives JSON. I want to validate that the request body is valid JSON and provide a valid error on the API response if not. Spent quite a lot of time trying different options and trying to find an answer but no success.

Whatever I do to try and catch the error, it always throws the following error when I send some incorrect test JSON.

JSON::ParserError at /api/v1/apikey123

743: unexpected token at '{
    "query": "hi there" (missing comma here on purpose)
    "lang": "en",
    "sessionId": "en" }

json (2.0.2) lib/json/common.rb, line 156

``` ruby
151     #   additions even if a matching class and create_id was found. This option
152     #   defaults to false.
153     # * *object_class*: Defaults to Hash
154     # * *array_class*: Defaults to Array
155     def parse(source, opts = {})
> 156       Parser.new(source, opts).parse  
157     end
158   
159     # Parse the JSON document _source_ into a Ruby data structure and return it.
160     # The bang version of the parse method defaults to the more dangerous values
161     # for the _opts_ hash, so be sure only to parse trusted _source_ documents.

Here is my code:

module Api
  class ApiController < ApplicationController
    protect_from_forgery with: :null_session
    before_action :authenticate, :parse_request

    private
    def parse_request
      begin
        @user_input = JSON.parse(request.body.read)
      rescue JSON::ParserError => e
         return false
      end
    end

    ...

  end
end

I'd like to know how to handle this without throwing an error and send back a response that has an error message with "Invalid JSON format"

I've just found that if I remove the Content-Type: application/json from the request headers, it works perfectly!! It seems that this is being called from somewhere else... is this normal Rails behaviour? — pronoob
– pronoob, Commented Dec 16, 2016 at 16:01

Alex Kojin · Accepted Answer · 2016-12-16 15:41:33Z

1

You should just call render method in your rescue block. The action method will be halted because you call render.

def parse_request
  begin
    @user_input = JSON.parse(request.body.read)
  rescue JSON::ParserError => e
    render json: {error: "Invalid JSON format"}, status: :unprocessable_entity
  end
end

answered Dec 16, 2016 at 15:41

Alex Kojin

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8 Comments

pronoob Over a year ago

Nice idea! However, I still get the above error before it gets to the rescue line :-(

pronoob Over a year ago

I've just found that if I remove the Content-Type: application/json from the request headers, it works perfectly!! It seems that this is being called from somewhere else... is this normal Rails behaviour?

Alex Kojin Over a year ago

If you send a request with json content-type then Rails middleware will parse it before to pass it in controller. So, when you remove content-type Rails will not be parse body to json. You should read this article about catching json errors robots.thoughtbot.com/…

Alex Kojin Over a year ago

I'm not sure but you can try to use rescue_from JSON::ParserError in controller.

Alex Kojin Over a year ago

This middleware is missed in Rails 5. You should another. Run rake middleware to list all. Probably you need to use Rack::Head

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