title: Adding devise gem to a Rails 4 / mysql project (table users already exists)
body: <p>I'm a total newbie with Ruby, Rails, Rake, MySQL (on Ubuntu/bash/command line) and <em>devise</em> (warden 1.2.3, devise 3.0.4). I must continue with what have been done so far : the Rails 4 project is working and linked to a MySQL database (full, populated with users and data).  I'm able to change controllers and views. </p>
  
  <p>I must add devise to the project.  I'm reading this : <a href="https://github.com/plataformatec/devise" rel="nofollow noreferrer">https://github.com/plataformatec/devise</a> . My class name is <em>User</em>. So I did <code>rails generate devise User</code>.</p>
  
  <p>When I do <code>rake db:migrate</code> I have this error : <code>Mysql2::Error: Table 'users' already exists: ...</code> of course, since I already have a <em>users</em> table in my project.</p>
  
  <p>Whatever I read ( <a href="https://stackoverflow.com/questions/16513066/devise-with-rails-4">Devise with Rails 4</a> ), it's always about <em>create the User model</em>, never <em>how to plug devise on a pre-existing User model</em>.</p>
  
  <p>What document should I read ? Which file I must edit to tell devise "this is my user model and table:" ?</p>