gistfile1.rb

  # This custom version of the authenticate method allows the user to use login or email as unique keys.
  # Please note that if using this method it's highly recommended that you make the email property, of the user model, a required one.
  # Also consider adding a unique index to the email field. For that create a migration with: add_index :users, :email, :unique => true
  # It's equally advisable to remove the nullable option from the email field.
  def self.authenticate(login, password)
    return nil if login.blank? || password.blank?
    u = find :first, :conditions => ['(email = ? OR login = ?) and activated_at IS NOT NULL', login, login] # need to get the salt
    u && u.authenticated?(password) ? u : nil
  end

Is it possible to delete or ignore the "login" field completely?

Certainly, you can change the behavior in whichever way you wish.

And which is the best way to achive that ? I mean, are there other pieces of code involved?