user.rb

class User < ActiveRecord::Base
  scope :email_exist, -> { where.not(email: nil) }

  attr_accessor :password              

  validates :nome, presence: true, length: { maximum: 50 } 

  before_save { self.email = email.downcase } 
  validates :email, presence: true, format: {with: /\A([^@\s]+)@((?:[-a-z0-9]+\.)+[a-z]{2,})\Z/i, on: :create}, uniqueness: {case_sensitive: false} 

  validates :password, presence: true, length: { minimum: 6 }


  def self.authenticate(email, password)
    user = email_exist.find_by(email: email)
    try(:authenticate, password)
  end

end

user_session.rb

class UserSession
    include ActiveModel::Model

    attr_accessor :email, :password
    validates_presence_of :email, :password

    def initialize(session, attributes={})
      @session = session
      @email = attributes[:email]
      @password = attributes[:password]
    end

    def authenticate!
      user = User.authenticate(@email, @password)

      if user.present?
        store(user)
      else
        errors.add(:base, :invalid_login)
        false
      end 
    end

    def store(user)
      @session[:user_id] = user.id
    end
end

user_sessions_controller.rb

class UserSessionsController < ApplicationController
  def new
    @user_session = UserSession.new(session)
  end
  
  def create
    @user_session = UserSession.new(session, params[:user_session])

    if @user_session.authenticate!
      redirect_to root_path, notice: t('flash.notice.signed_in')
    else
      render :new
    end
  end

  def destroy
  end 

end