如何为nil捕获“未定义的方法`[]”：NilClass“错误？

Question

I get an nested array from facebook via omniauth and wanna check if it's empty?/nil?/exists? the depending line looks like:

 unless omniauth['extra']['raw_info']['location']['name'].nil?

This should check if this part of the array is empty or exists.

But always this error was thrown:

undefined method `[]' for nil:NilClass

Do I check arrays wrong?

I tried it with "has_key" "nil?" "empty?" "exists?" "blank?"

But no one of these works!

Please help me, many thanks in advance!

Answer 1

This error is raised because one of the hash values in the chain of omniauth['extra']['raw_info']['location']['name'].nil? returns nil and it is not the last call ['name'].

If for example omniauth['extra']['raw_info'] returns nil, you're actually trying to call nil['location'] which raises an error in ruby.

You can catch this error simply:

res = omniauth['extra']['raw_info']['location']['name'].nil? rescue true

unless res
  #your code here
end

Please notice that the code block above will fill the variable res with true if the ['name'] hash value is nil or any other hash value in the chain returns nil.

Answer 2

Ideally you should check each nested level to see if it is nil , however, this will also work.

unless (omniauth['extra']['raw_info']['location']['name'] rescue nil).nil?

You can also rescue the NoMethodError specifically.

Answer 3

A bit late to the party, but, as pointed in this answer , Ruby 2.3.0 introduced a new method called dig , which would return nil if one of the chained keys is nil . Your omniauth auth hash could then be presented as:

omniauth = { 
            ...                 
            "extra"=>{ "raw_info"=>
                        { "location"=>"New York",
                          "gravatar_id"=>"123456789"}}
             ...
           }


omniauth.dig('extra', 
             'raw_info',
             'location',
             'name',
             'foo',
             'bar',
             'baz') #<= nil