How to test Controller post :create of JSON api on

I have been tearing my hair trying to make the test to pass. I have a JSON API that looks like this:

{
  "data": [
    {
      "id": "b99f8173-0492-457f-9de9-6c1d8d6832ed",
      "type": "manufacturer_organizations",
      "attributes": {
        "account_number": "random test 123"
      },
      "relationships": {
        "organization": {
          "data": {
            "id": "fb20ddc9-a3ee-47c3-bdd2-f710541ff89c",
            "type": "organizations"
          }
        },
        "manufacturer": {
          "data": {
            "id": "1",
            "type": "manufacturers"
          }
        }
      }
    },...

I am trying to make a post :create test in rails.

let!(:manufacturer_organization) {FactoryGirl.create(:manufacturer_organization)}
let(:manufacturer_organization2) { FactoryGirl.create(:manufacturer_organization)}

...

  describe "POST create" do
    it "posts a valid manufacturer organization data" do
      authenticate
      organization = FactoryGirl.create(:organization)
      manufacturer = FactoryGirl.create(:manufacturer)

      post :create, manufacturer_organization2.to_json #what should I put here instead?

      expect(json['data'].length).to eq(2)
    end

  #=> error: JSON::ParserError: A JSON text must at least contain two octets!

I have tried various SO posts (this), this, and this article

Here are some of the attempts I have tried:

post :create, params: {organization_id: organization.id, manufacturer: manufacturer.id, account_number: "123 test account number"}
#=> error: JSON::ParserError:
   A JSON text must at least contain two octets!

post :create, params: :manufacturer_organization_2
#=> 
 NoMethodError:
   undefined method `symbolize_keys' for :manufacturer_organization_2:Symbol

json = { :format => 'json', :manufacturer_organization => { :account_number => "foo123", :organization_id => organization.id, :manufacturer_id => manufacturer.id } }
post :create, json
#=>  NoMethodError:
   undefined method `length' for nil:NilClass

How can I test my controller to accept manufacturer_id, organization_id, and account_number via post :create? Right now the way I test it is to count initial json['data'].length (initially 1); at the end I expect json['data'].length to be 2 after successful post :create. How can I mock creating a valid manufacturer_organization input?

Edit:

Sorry, forgot to put my json method helper:

def json
  JSON.parse(response.body)
end

Also, this pass:

  describe "POST create" do
    it "posts a valid manufacturer organization data" do
      authenticate
      organization = FactoryGirl.create(:organization)
      manufacturer = FactoryGirl.create(:manufacturer)
      post :create, {account_number: "Test account numba", organization_id: organization.id, manufacturer_id: manufacturer.id}
      expect(response).to be_success
    end

while adding expect(json['success']).to eq("Yay!") gives me this error:

JSON::ParserError: A JSON text must at least contain two octets!

Controller:

  def create
    @manufacturer_organization = ManufacturerOrganization.new(manufacturer_organization_params)
    if @manufacturer_organization.save
      puts "success!"
      render json: {success: "Yay!"}
    else
      puts "Sorry, something went wrong!"
    end
  end


def manufacturer_organization_params
    api_params.permit(:organization_id, :manufacturer_id, :account_number)
end

while @api_params ||= ActionController::Parameters.new(ActiveModelSerializers::Deserialization.jsonapi_parse(params))

In RSpec you never* need to explicitly format the params.

post :create, params: { foo: 'bar' }, format: :json

This will properly format the hash { foo: 'bar' } as JSON in the request body.

To create a hash which matches the JSONAPI.org structure you can create a helper:

# support/api_spec_helper.rb
module APISpecHelper
  def to_json_api(model)
    {
      data: {
        type: ActiveModel::Naming.plural(model),
        attributes: model.attributes
      }.tap do |hash| 
        hash[:id] = model.id if model.persisted?
      end
    }
  end
end

You can also use the JSONAPI-RB gem or ActiveModel::Serializers to constuct/deconstruct JSONAPI responses/params.

require 'rails_helper'
require 'api_spec_helper'

RSpec.request "Manufacturer organizations" do 

  include APISpecHelper

  describe "POST '/manufacturer_organizations'" do
    let(:valid_params) do
      to_json_api(FactoryGirl.build(:manufacturer_organization))
    end
    let(:invalid_params) do
      to_json_api(ManufacturerOrganization.new(
        foo: 'bad_value'
      ))
    end

    describe "with valid attributes" do
      it "creates a manufacturer organization" do
        expect do
          post '/manufacturer_organizations', params: valid_params, format: :json
        end.to change(ManufacturerOrganization, :count).by(+1)
      end

      it "has the correct response" do
        post '/manufacturer_organizations', params: valid_params, format: :json
        expect(response).to have_status :created
        expect(response.headers[:location]).to eq(
          manufacturer_organization_path(ManufacturerOrganization.last)
        )
      end
    end

    describe "with valid attributes" do
      it "does not create a manufacturer organization" do
        expect do
          post '/manufacturer_organizations', params: invalid_params, format: :json
        end.to_not change(ManufacturerOrganization, :count)
      end

      it "has the correct response" do
        post '/manufacturer_organizations', params: invalid_params, format: :json
        expect(response).to have_status :unproccessable_entity
      end
    end
  end
end

Returning the correct status codes.

Returning the correct response codes is pretty simple:

def create
  @resource = Resource.create(some_params)
  if @resource.save
    # you can respond by pointing at the newly created resource but with no body
    head :created, location: @resource
    # or 
    render json: @resource, 
           status: :created, 
           location: @resource
  else
    render json: { errors: @resource.errors.full_messages }, 
           status: :unprocessable_entity
  end
end

If a POST request did not include a Client-Generated ID and the requested resource has been created successfully, the server MUST return a 201 Created status code.
http://jsonapi.org/format/#crud

Other Responses
A server MAY respond with other HTTP status codes.
A server MAY include error details with error responses.

The commonly accepted practice is to use 422 - Unprocessable Entity for validation errors.

One small concern is that you should use a serializer to give the correct JSON response and also serialize the correct error objects.