Instruct Rails to respect the Accept header even if it contains a wildcard

api_controller.rb

class ApiController < ApplicationController
  before_action :only_respect_accept_header

  private

  # By default, Rails will ignore the Accept header if it contains a wildcard
  # and assume the client wants HTML (or JS if using XMLHttpRequest). See
  # https://github.com/rails/rails/blob/a807a4f4f95798616a2a85856f77fdfc48da4832/actionpack/lib/action_dispatch/http/mime_negotiation.rb#L171-L173
  #
  # If you don't expect your clients to be browsers, we want to override this
  # and only set the request formats from the Accept header, falling back to
  # the wildcard if it is not present (and therefore respecting our
  # priorities).
  #
  # Note we have to filter down the MIME types to ones Rails understands
  # otherwise it will raise an error when attempting to set formats for the
  # lookup context.
  def only_respect_accept_header
    request.set_header(
      "action_dispatch.request.formats",
      requested_mime_types.select { |type| type.symbol || type.ref == "*/*" }
    )
  end

  def requested_mime_types
    Mime::Type.parse(request.get_header("HTTP_ACCEPT").to_s).presence || [Mime::ALL]
  end
end

api_controller_spec.rb

require "rails_helper"

RSpec.describe ApiController do
  controller do
    def index
      respond_to do |format|
        format.text { render plain: "Plain text" }
        format.json { render json: "JSON" }
      end
    end
    
    def show
      respond_to do |format|
        format.text { render plain: "Plain text" }
        format.any { render plain: "Fallback" }
      end
    end
  end

  it "returns the first format when given no Accept header" do
    get :index

    expect(response.media_type).to eq("text/plain")
  end

  it "returns the first format when given a wildcard Accept header" do
    request.headers["Accept"] = "*/*"

    get :index

    expect(response.media_type).to eq("text/plain")
  end

  it "returns the first format when given Safari and Chrome's default Accept header" do
    request.headers["Accept"] = "text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,image/apng,*/*;q=0.8"

    get :index

    expect(response.media_type).to eq("text/plain")
  end

  it "returns a text/plain response when explicitly requested" do
    request.headers["Accept"] = "text/plain"

    get :index

    expect(response.media_type).to eq("text/plain")
  end

  it "returns an application/json response when explicitly requested" do
    request.headers["Accept"] = "application/json"

    get :index

    expect(response.media_type).to eq("application/json")
  end

  it "raises an error if given an unsupported MIME type" do
    request.headers["Accept"] = "image/webp"

    expect { get :index }.to raise_error(ActionController::UnknownFormat)
  end
  
  it "does not raise an error when given an unsupported MIME type if an 'any' format is present" do
    request.headers["Accept"] = "image/webp"

    get :show, params: { id: 1 }

    expect(response.body).to eq("Fallback")
  end

  it "raises an error when given an invalid MIME type" do
    request.headers["Accept"] = "not a valid MIME type"

    expect { get :index }.to raise_error(Mime::Type::InvalidMimeType)
  end

  it "raises an error when given an invalid MIME type even if an 'any' format is present" do
    request.headers["Accept"] = "not a valid MIME type"

    expect { get :show, params: { id: 1 } }.to raise_error(Mime::Type::InvalidMimeType)
  end
end