Question

repeat_purchases_percent is a method in my Category model. All parentless categories are guarenteed to have a value for future_purchases_percent in the database. Subcategories may or may not have a future_purchases_percent. if they do, i want to use that value. If not, I want to use the parent’s value and so on.

I tried a recursive method.

def future_purchases_percent
  if self.future_purchases_percent.nil?
    Category.find(self.parent_id).future_purchases_percent
  else
    future_purchases_percent
  end
end

This gets stuck in a loop where it keeps evaluating:

if self.future_purchases_percent.nil?

how can i correctly implement this method?

Answer 1

1. Your method gets stuck because it has the same name than the attribute (so it’s calling itself), pick a different name.
2. Notice that this conditional is nothing more than a simple a || b expression.
3. You should add a belongs_to :parent, :foreign_key => 'parent_id', :class_name => :Category to this model.

Now, with everything in place, you can write real declarative code:

def safe_future_purchases_percent
  future_purchases_percent || parent.safe_future_purchases_percent
end