Why is the "||" symbol required in stock_count method? Why isn't it simply "@stock_count = 0?"

This is going to sound a little pedantic, but be careful with your terminology... There is no || operator present in the code, there's a ||= operator. ||, ||=, and = are three separate operators with three separate meanings.

= will assign a new value to a variable, regardless of whether it already contains a value. ||= will assign a new value only if the variable's current value is nil.

So if we wrote this:

def stock_count
  @stock_count = 0
end

Then every time we called stock_count, the value of @stock_count would be reset to 0, and the stock_count method would always return 0. Instead, we write:

def stock_count
  @stock_count ||= 0
end

So if a value has never been assigned to @stock_count before, the stock_count method will assign it 0 and return 0. Otherwise (apparently) it will just return the current value of @stock_count.

It should be noted that this is a slightly unusual way of handling an attribute reader method. Normally, the method would just look like this:

def stock_count
  @stock_count
end

...and @stock_count would be set to 0 in the initialize method.

Thank you Jay