Question

This method increments the ActiveRecord attr_accessible attribute current_step:

def next_step
  logger.debug "Now at step: " + current_step.inspect
  if (current_step == nil)
    current_step = 0
  end
  current_step = current_step + 1
end

At execution of the method, the log shows Now at step: 0 but the +1 line fails:

NoMethodError (You have a nil object when you didn't expect it!
You might have expected an instance of Array.
The error occurred while evaluating nil.+):
  app/models/assessment.rb:16:in `next_step'

Is it a miracle? Is current_step nil or not?

---

EDIT: Thanks fl00r and whitequark for the great answers! Here is what the code was meant to be:

def next_step
  current_step ||= 0
  self.current_step += 1
  save
end

Answer 1

Conditionals and loops do not have their own scope in Ruby; and here you have a variable and a method of self which possess the same name. In the condition of if, the current_step method is used, but in its body a local variable is defined, and all future references to the current_step will refer to the local variable. The pitfall you’ve encountered is, even if the if body is not executed, the local variable is still defined, and default value of nil is assigned to it.

I’ll make it more clear by adding _M to the identifier when the method is accessed, and _L for the local variable.

def next_step
  logger.debug "Now at step: " + current_step_M.inspect
  if (current_step_M == nil)
    current_step_L = 0
  ### this part is implicit:
  # else
  #   current_step_L = nil
  end
  current_step_L = current_step_L + 1
end

I’m guessing you was actually trying to do self.current_step = 0, which would invoke the setter.