防止未定义的链式方法
我有一个很长的循环，结果是这样的：
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
该方法使用产品 1-17 效果很好，导致打印出 url。当我到达第 18 条记录时，我遇到了问题
Product.find(18) // product found!
product.find(18).page_url("en_US")
NoMethodError: undefined method `page_url' for nil:NilClass
如何防范这些未定义事件？
url = 产品.page_url("en_US")
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")
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评论（2）
问题是
product为nil：它与
page_url可能返回nil 没有任何关系 code>。）确保
product不能为nil：但要小心，这可能是一个更深层次的问题。无论如何，“修复”这个问题很容易。 。考虑使用集合 限制（例如 Enumerable#reject）：
上面使用Symbol#to_proc“Rails 魔法”，但可以与限制一样，
{|x| x.nil?}的缺点是，尽管Enumerable#partition 可以帮助解决这个问题：使用正确的工具来完成这项工作。另一个解决方案是扩大条件检查本身：
&&的短路性质将确保page_url仅在真值时调用（不包括nil）。我还冒昧地假设
page_url无法返回false，因为我发现这使得意图更加清晰。快乐编码。
The issue is that a
productisnil:(It has nothing to do with
page_urlmaybe returningnil.)Make sure
productcan't benil: 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):
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:
The short-circuit nature of
&&will ensurepage_urlis only invoked upon a truthy value (which excludesnil).I also took the liberty of assuming
page_urlcan't returnfalseas I find this makes the intent more clear.Happy coding.
试试这个：
但它是性能杀手。
您确定
Product.find(18)不会返回nil吗？无论如何，你可以这样做：
Try this:
But it's a perf killer.
Are you sure
Product.find(18)doesn't returnnil?Anyway, you could do: