Protecting against an undefined chained method

Question

I have a long loop that results in this:

csv_code = CSV.generate do |csv|
  csv << ["Product ID","Name", "Url"]
  @all_products.each do |product|
      if product.page_url("en_US") != nil
      turl = product.page_url("en_US")
      end
    csv << [product.name,product.d_id, turl]
  end
end

The method uses products 1-17 works great resulting in a url printed. When I get to my 18th record I have problems

Product.find(18) // product found!
product.find(18).page_url("en_US")
NoMethodError: undefined method `page_url' for nil:NilClass

How can I protect against these undefined events?

url = product.page_url("en_US")

When I have an undefined method, I need to protect against it so the loop continues. — JZ., Sep 19 '11 at 20:45
related: http://stackoverflow.com/questions/4565496/ruby-syntactic-sugar-dealing-with-nils. May advice: ick's maybe. — tokland, Sep 19 '11 at 20:47
The issue is that a `product` is `nil`: "undefined method 'page_url' for nil:NilClass". Solution: Make sure `product` can't be `nil` (this may be a deeper issue, but it's easy to filter out -- consider `Enumerable.select` of `if product && product.page_url ...`, etc). — , Sep 19 '11 at 20:49

score 3 · Accepted Answer · 2011-09-19T21:07:54.343

The issue is that a product is nil:

undefined method 'page_url' for nil:NilClass". Solution:

(It has nothing to do with page_url maybe returning nil.)

Make sure product can't be nil: but be wary that this may be a deeper issue. In any case, "fixing" this issue is easy to deal with.

Consider either using a collection restriction (such as Enumerable#reject):

@all_products.reject(&:nil?).each do {
   ...
}

The above uses the Symbol#to_proc "Rails magic", but could just as easily have been {|x| x.nil?} as the restriction. The downside is it's not practical to use this for a "no URL" condition per-product although Enumerable#partition could help with that: use the right tool for the job.

Another solution is to expand the conditional check itself:

if product && product.page_url("en_US")
   # yay
else
   # uhm
end

The short-circuit nature of && will ensure page_url is only invoked upon a truthy value (which excludes nil).

I also took the liberty of assuming page_url can't return false as I find this makes the intent more clear.

Happy coding.

score 2 · Answer 2 · answered Sep 19 '11 at 20:45

2

Try this:

product.find(18).try(:page_url, "en_US")

But it's a perf killer.

Are you sure Product.find(18) doesn't return nil ?

Anyway, you could do:

url = product.nil? ? "no_url" : product.page_url("en_US")

answered Sep 19 '11 at 20:45

apneadiving

114,565
26
219
213

the problem is the locale is "en_CA" – JZ. Sep 19 '11 at 20:50
ok so `try` should be ok but what you wrote in your question doesn't lead to this conclusion – apneadiving Sep 19 '11 at 20:54

Protecting against an undefined chained method

2 Answers2