A snippet of Monoceros code

_form.html.erb

<%= form_tag monoceros_sessions_path do %>
  <p>
    <%= label_tag :username_or_email %><br />
    <%= text_field_tag :login, params[:login] %>
  </p>
  <p>
    <%= label_tag :password %><br />
    <%= password_field_tag :password %>
  </p>
  <p class="button"><%= submit_tag "Log in" %></p>
<% end %>

Monoceros::Authentication::SessionsController.eb

class Monoceros::Authentication::SessionsController < ApplicationController
  def new

  end

  def create
    user = User.authenticate(params[:login].downcase, params[:password])
    if user
      session[:user_id] = user.id
      redirect_to monoceros_dashboard_url, :notice => "Logged in!"
    else
      flash.now.alert = "Invalid email or password"
      render "new"
    end
  end

  def destroy
    session[:user_id] = nil
    redirect_to monoceros_root_url, :notice => "Logged out!"
  end
end