Question

Confused by this method – can someone please explain it to me?

def current_user
    @current_user ||= (login_from_session || login_from_cookie) unless @current_user == false
end

Answer 1

It says:

1. unless current is not set (the latest unless)
2. do nothing and return @current_user if @current_user is already set (the ||= part)
3. else call the method/helper login_from_session and assign the result to @current_user
4. else if the previous call has returned nil or false, call the method/helper login_from_cookie and assign the result to @current_user
5. in any case return the value of @current_user instance variable

It could be rewritten to be more explicit in this way

def current_user
  if !(@current_user == false) # 1
    if (@current_user)
      return @current_user # 2
    end
    if (@current_user = login_from_session)
      return @current_user # 3
    end
    if (@current_user = login_from_cookie)
      return @current_user # 4
    end  
  end
  return @current_user # 5
end

This is the power (and the beauty) of ruby expressiveness. Remember that in Ruby only nil and false evaluate to boolean false in if/else statements and ||, && operators

Other hints to understand better, in ruby you have the following rules:

The return value of any function is the last expression evaluated for the function, so

def foo
  any_value
end

is the same of

def foo
  return any_value
end

The if/unless statement at the end of an expression is the same of a if/unless statement, so

do something if value

is the same of

if (value)
  do_something
end

The ||= operator is a shortcut for

@a ||= some_value
# is equivalent to
if !@a
  @a = some_value
end

Combine all these rules and you’ve the method explained.