## In my User model

def password=(arg)
    arg = MD5.new(arg).hexdigest unless arg.length == 32
    self.password = arg
end

## When testing in script/console

>> @user = User.new
=> #<User:0x35300cc @new_record=true, @attributes={"username"=>"", "password"=>""}>
>> @user.password = 'test'
SystemStackError: stack level too deep
        from ./script/../config/../config/../app/models/user.rb:35:in `password='
        from ./script/../config/../config/../app/models/user.rb:36:in `password='
        from (irb):2