Topic: How to convert PHP AJAX code to Ruby/Rails

I'm converting a PHP website to Ruby/Rails. I have a form and when a User signs up he is asked for an email address among other things. I check if the email is a duplicate via AJAX. So my only field in the form which requires an AJAX check is the email field.  I set up the JavaScript like so:

function checkEmail()
{
  $.ajaxSetup({async:false});
  $.post('duplicateEmail.php', {email: $('#email').val()}, 
  function(data)
  {
    if(!data.success)
    {
      $('#email').addClass('emptyField');
      setError('error');
      setError('dupEmail');
    }
    else if(data.success)
    {
      $('#email').removeClass('emptyField');
      resetError('dupEmail');
    }
  },
'json');
}

Then in a PHP file I have the following:

$email = $_POST['email'];

$query = "SELECT id FROM users WHERE email='$email'";
$result = $mysqli->query($query);

$data['error'] = mysql_error();

if($result->num_rows == 0)
{
  $result->free();
  $data['success'] = true;
  $data['message'] = '';
}
else
{
  $result->free();
  $data['success'] = false;
  $data['message'] = 'This email address has already been registered.';
}

echo json_encode($data);

Can anyone point me in the right direction as to where to place the Ruby code?  Or am I thinking about this in the wrong way?