How To: Find a user when you have their credentials

(I can't believe how frustratingly difficult it was to find this, so I'm recording the answer here for posterity.)

From https://github.com/plataformatec/devise/issues/346, you can do the following:

user = User.find_for_authentication(:username => 'druidia')
user.valid_password?('12345')  # That's amazing! I have the same combination on my luggage!

Here's a convenience method for same:

class User
  def self.authenticate(username, password)
    user = User.find_for_authentication(:username => username)
    user.valid_password?(password) ? user : nil
  end
end