```ruby
require 'factory_girl'
require 'rails_helper'

describe SessionsController do
  describe '#login' do
    context 'with valid credentials' do
      before :each do
        @user = FactoryGirl.create(:user)
        post :create, user: @user.attributes
      end

      it 'sets session[:id] to the user\'s id' do
        expect(session[:id]).to eq(@user.id)
      end

      it 'redirects to root_url upon successful login' do
        expect(response).to redirect_to root_url
      end
    end

    context 'without valid credentials' do
      before :each do
        @invalid_user = User.create(name: '', password: '')
        post :create, user: @invalid_user.attributes
      end

      it 'renders the login page' do
        expect(response).to render_template('sessions/login')
      end

      it 'displays an error message for users with invalid credentials' do
        # TODO: write test once error helpers are defined
      end
    end
  end

  describe '#logout' do
    before :each do
      @user = FactoryGirl.create(:user)
      post :create, user: @user.attributes
    end

    it 'clears the session[:id]' do
      expect(session[:id]).to eq(@user.id)
      get :logout, id: @user
      expect(session[:id]).to eq(nil)
    end

    it 'redirects the user to the root_url' do
      get :logout, id: @user
      expect(response).to redirect_to root_url
    end
  end
end
```

Remove error message controller test, will write a feature test for it.
user
```ruby
require 'factory_girl'
require 'rails_helper'

describe SessionsController do
  describe '#login' do
    context 'with valid credentials' do
      before :each do
        @user = FactoryGirl.create(:user)
        post :create, user: @user.attributes
      end

      it 'sets session[:id] to the user\'s id' do
        expect(session[:id]).to eq(@user.id)
      end

      it 'redirects to root_url upon successful login' do
        expect(response).to redirect_to root_url
      end
    end

    context 'without valid credentials' do
      before :each do
        @invalid_user = User.create(name: '', password: '')
        post :create, user: @invalid_user.attributes
      end

      it 'renders the login page' do
        expect(response).to render_template('sessions/login')
      end
    end
  end

  describe '#logout' do
    before :each do
      @user = FactoryGirl.create(:user)
      post :create, user: @user.attributes
    end

    it 'clears the session[:id]' do
      expect(session[:id]).to eq(@user.id)
      get :logout, id: @user
      expect(session[:id]).to eq(nil)
    end

    it 'redirects the user to the root_url' do
      get :logout, id: @user
      expect(response).to redirect_to root_url
    end
  end
end
```
assistant