Question

In Michael Hartl’s tutorial he has a module SessionsHelper, which has a method called current_user= which accepts a parameter and assigns it to @current_user. He then invokes this in various other methods in his program using a line like current_user = User.first.

My question is how we know that the current_user= method is being called at all — it seems to me that what would actually be happening is a new variable called current_user is created on the spot, given the value of User.first, and then when the function closes that variable dies.

I also tested this simply by creating the following code:

def x= val
puts "method called"
end

x = 46
puts x

This code snippet simply prints 46 on the screen – the function x= is never called at all. So what is the current_user= method doing in Michael Hartl’s tutorial, and how does he cause that function to be called?

Answer 1

It’s because Ruby treat expressions that can be read as simple variable as simple variable. You can see it with this code:

def x
  10
end

print x      #=> 10
x = 5
print x      #=> 5
print x()    #=> 10
print self.x #=> 10

This is the same case. Look:

def x=(a)
  print a
end

x = 8      #=> Variable 'x' assigned to 8
x=(8)      #=> The same

self.x = 8 #=> Function called

Also remember that assignment functions (like the last one) can’t return any value, the return value will always be the last argument.

def x=(a)
  return a+1
end

var = (self.x = 8) #=> Function called; Variable 'var' assigned to 8