How do I unit test the current_user RoR

I have this method to check if the user is admin:

def admin?
  current_user.admin == true
end

The unit test is:

require 'rails_helper'

describe StubController do
  describe '.admin?' do
    it "should tell if the user is admin" do
      user = User.create!(email: "i@i.com", password:'123456', role: "admin", name: "Italo Fasanelli")

      result = user.admin?

      expect(result).to eq true
    end
  end
end

The problem is, simplecov is telling me that this part current_user.admin == true is not covered.

How do I test the current_user in this test?

First off, move the admin? method to User model so that it can be reused across Model-View-Controller.

class User < ApplicationRecord
  def admin?
    role == 'admin'
  end
end

You can use this method wherever you have access to the instance of User. So current_user.admin? would also work across views and controller.

Now you should write test for model not the controller. Also I noticed that you create user model object manually instead of using Factory. Use FactoryBot to create required instances for testing.

Here is a quick spec assuming there is factory is set for user

require 'rails_helper'

RSpec.describe User, type: :model do
  describe '.admin?' do
    context 'user has role set as admin' do
      let!(:user) { build(:user, role: 'admin') }

      it 'returns true' do
        expect(user).to be_admin
      end
    end

    context 'user has role set as non admin' do
      let!(:user) { build(:user, role: 'teacher') }

      it 'returns true' do
        expect(user).not_to be_admin
      end
    end
  end
end