note solution in this article works in Rails 5 (tested), Rails 6 (tested) Rails 7 (tested)

Imagine you generate Rails API only app (rails new --api) and now you want all the requset been considered as JSON content-type (even if the header is not present in the request)

So when you do:

curl -XPOST localhost:3000/email_authentication -d '{"email": "tomas"}'

… it would be same as doing:

curl -XPOST localhost:3000/email_authentication -d '{"email": "tomas"}' -H 'Content-Type: application/json'

Rails by default has content-type: application/x-www-form-urlencoded

Now you may see some tutorials that poins you to just do:

 # app/controllers/application_controller.rb
 class ApplicationController < ActionController::Base
		respond_to :json
		before_action :set_default_response_format

		private

		def set_default_response_format
			request.format = :json
		end
  # ...

BUT THIS WILL NOT WORK IN Rails API only app. It seems there is some middleware missing.

Long story short you can create a middleware that will translate default Rails header to be Content-Type: application/json for requests:

# config/application.rb
# ...
require './lib/middleware/consider_all_request_json_middleware'
# ...

module MyApplication
	# ...
  class Application < Rails::Application
		# ...
		config.middleware.insert_before(ActionDispatch::Static,ConsiderAllRequestJsonMiddleware)
		# ...

# lib/middleware/consider_all_request_json_middleware.rb
class ConsiderAllRequestJsonMiddleware
  def initialize app
    @app = app
  end

  def call(env)
    if env["CONTENT_TYPE"] == 'application/x-www-form-urlencoded'
      env["CONTENT_TYPE"] = 'application/json'
    end
    @app.call(env)
  end
end

what if I don’t want api only

You may want to limit it only for certain path e.g. /api/anything while keeping the rest of the app as it is

class ConsiderAllRequestJsonMiddleware
  def initialize app
    @app = app
  end

  def call(env)
    if env['PATH_INFO'].match(/\A\/api\/.*/) # if match /api/*
      if env["CONTENT_TYPE"] == 'application/x-www-form-urlencoded'
        env["CONTENT_TYPE"] = 'application/json'
      end
    end

    @app.call(env)
  end
end

Notes for myself

In normal circumstances browser don’t send any header with GET but sends header “application/x-www-form-urlencoded” when POST PUT

When browser submits a form with file it sends header “multipart/form-data”

Rails will parse request.body to params according to Content-Type. So when you use value application/json body {"hello":"value"} will end up parsed to params while with default application/x-www-form-urlencoded it will end up ignored

So that’s why FE/Browser don’t necessary needs to send Content-Tye header with GET but definitelly needs to send header Content-Type=application/json with POST PUT data submissions to API (beside other things Rails knows how that it needs to parse request.body to JSON)

JSON is not designed to send files in the format - we cannot do {"file":"x03...BINARY....x4O"}

well we could but it would have to involve some FE voodoo converting file to binary string == not worh it

That’s why when submitting files to JSON API the format should stay “multipart/form-data”