RSpec undefined method `[]' for nil:NilClass in sessions controller spec - ruby-on-rails

Question

I've got a simple message app to learn RSpec where one user can create message to another user (only logged users can write messages). I didn't used devise to log in or sign up users, this app is as simple as possible just for rspec learning.
I want to test create method in sessions controller:
class SessionsController < ApplicationController
before_action :logged_in_redirect, only: %i[new create]
def new; end
def create
user = User.find_by(username: params[:session][:username])
if user && user.authenticate(params[:session][:password])
session[:user_id] = user.id
flash[:success] = 'You have successfully logged in'
redirect_to root_path
else
flash.now[:error] = 'There was something wrong with your login'
render 'new'
end
end
def logged_in_redirect
if logged_in?
flash[:error] = 'You are already logged in'
redirect_to root_path
end
end
This is relevant code from my sessions controller spec:
RSpec.describe SessionsController, type: :controller do
describe 'POST #create' do
context 'when password is correct' do
let(:user) { User.create(username: 'John', password: 'test123') }
it 'redirect to root path' do
post :create, session: { username: 'John', password: 'test123' }
expect(response).to have_http_status(:redirect)
end
end
end
end
The error says undefined method `[]' for nil:NilClass. It works as expected in the application. What am I missing? Should I use session[:user_id] after let(:user) ?

Answer 1

Use let to define a memoized helper method. The value will be cached
across multiple calls in the same example but not across examples.
Note that let is lazy-evaluated: it is not evaluated until the first time the method it defines is invoked. You can use let! to
force the
- List item
method's invocation before each example.
xou can refer this to get further detailsrelish docs
Possibility 1
RSpec.describe SessionsController, type: :controller do
describe 'POST #create' do
before :each do
#user = User.create(username: 'John', password: 'test123')
end
context 'when password is correct' do
it 'redirect to root path' do
post :create, params: {session: { username: #user.username, password: #user.password }}
expect(response).to have_http_status(:redirect)
end
end
end
end
Possibility 2
RSpec.describe SessionsController, type: :controller do
let(:create_user) do
#user = User.create(username: 'John', password: 'test123')
end
describe 'POST #create' do
context 'when password is correct' do
it 'redirect to root path' do
create_user
post :create, params: {session: { username: #user.username, password: #user.password }}
expect(response).to have_http_status(:redirect)
end
end
end
end