class UsersController < ApplicationController

  def index
    @users = User.all
  end

  def new
    @user = User.new
  end

  def create
    @user = User.new(user_params)
    if @user.save
      session[:user_id] = @user.id
      redirect_to @user
    else
      redirect_to :back
    end
  end

  def show
    @user = User.find(params[:id])
  end

  def edit
    @user = User.find(params[:id])
  end

  def update
    @user = User.new(user_params)
    if @user.save
      redirect_to @user
    else
      redirect_to 'edit_user_path'
    end
  end

  private
  def user_params
    params.require(:user).permit(:first_name, :last_name, :username, :city, :state, :email, :password)
  end

end

class UsersController < ApplicationController

  def index
    @users = User.all
  end

  def new
    @user = User.new
  end

  def create
    @user = User.new(user_params)
    if @user.save
      session[:user_id] = @user.id
      redirect_to @user
    else
      redirect_to :back
    end
  end

  def show
    @user = User.find(params[:id])
  end

  def edit
    @user = User.find(params[:id])
  end

  def update
    @user = User.new(user_params)
    if @user.update(params[:user])
      redirect_to @user
    else
      @errors = @user.errors.full_messages
      redirect_to 'edit_user_path'
    end
  end

  private
  def user_params
    params.require(:user).permit(:first_name, :last_name, :username, :city, :state, :email, :password)
  end

end

Add error for failed update request
Add error for failed update request
