login_and_authenticate.rb

#if using has_secure_password in model
#use password_digest and confirm_password in migration
#otherwise password_hash in migration

#CONTOLLER

get '/' do
  # Look in app/views/index.erb
  erb :index
end

post '/' do
  if User.authenticate(params[:username], params[:password])
   @user = User.find_by_username(params[:username])
    session[:user_id] = @user.id  #creates session
    redirect "/user/#{current_user.id}"
  else
    redirect '/'
  end
end

#MODEL

class User < ActiveRecord::Base
  include BCrypt
  def password
    @password ||= Password.new(password_hash)
  end

  def password=(pass)
    @entered_password = pass
    @password = Password.create(pass)
    self.password_hash = @password
  end

  def self.authenticate(email, password)
    user = User.find_by_email(email)
    return user if user && (user.password == password)
    nil # either invalid email or wrong password
  end
end
  
#INDEX VIEW
<div class="container">
  <div id="sign_in">
    <h1>Sign in</h1>
    <form method="post" action="/">
      <div class="username">
        <label>username</label></br>
        <input type="text" name="username">
      </div>
      <div class="password">
        <label>password</label></br>
        <input type="password" name="password">
      </div>
      <div class="submit">
        <input type="submit" value="Submit" class="submit_button">
      </div>
        <h2>or</h2>
        <div id="create">
          <a href="/user/new">Create Account</a>
        </div>
    </form>
  </div>
</div>

#CREATE ACCOUNT VIEW

<div class="container">
  <div id="create_account">
    <h1>Create account</h1>
    <form method="post" action="/create_account">
      <div class="username">
        <label>username</label></br>
        <input type="text" name="user[username]">
      </div>
      <div class="password"
        <label>password</label></br>
        <input type="password" name="user[password]">
      </div>
      <div class="submit">
        <input type="submit" value="Create" class="submit_button">
      </div>
    </form>
  </div>
</div> 




This is a nice helper method to consider including in a helper file.

helpers do

  def current_user
   @current_user ||= User.find_by_id(session[:user_id])
  end

end
Other things I'm throwing on here so that all Bcrypt-esque items are in one place...don't forget to:

require 'bcrypt' in your environment (or in the User model, above/outside of class User)
include the bcrypt gem in your gemfile (gem 'bcrypt')
and to bundle after you've made sure to include the bcrypt gem in your gemfile
...in addition to calling include BCrypt inside of your User model