Ruby – Access multidimensional hash and avoid access nil object

conditionalhashruby

Possible Duplicate:
Ruby: Nils in an IF statement
Is there a clean way to avoid calling a method on nil in a nested params hash?

Let's say I try to access a hash like this:

my_hash['key1']['key2']['key3']

This is nice if key1, key2 and key3 exist in the hash(es), but what if, for example key1 doesn't exist?

Then I would get NoMethodError: undefined method [] for nil:NilClass. And nobody likes that.

So far I deal with this doing a conditional like:

if my_hash['key1'] && my_hash['key1']['key2'] …

Is this appropriate, is there any other Rubiest way of doing so?

Best Solution

There are many approaches to this.

If you use Ruby 2.3 or above, you can use dig

my_hash.dig('key1', 'key2', 'key3')

Plenty of folks stick to plain ruby and chain the && guard tests.

You could use stdlib Hash#fetch too:

my_hash.fetch('key1', {}).fetch('key2', {}).fetch('key3', nil)

Some like chaining ActiveSupport's #try method.

my_hash.try(:[], 'key1').try(:[], 'key2').try(:[], 'key3')

Others use andand

myhash['key1'].andand['key2'].andand['key3']

Some people think egocentric nils are a good idea (though someone might hunt you down and torture you if they found you do this).

class NilClass
  def method_missing(*args); nil; end
end

my_hash['key1']['key2']['key3']

You could use Enumerable#reduce (or alias inject).

['key1','key2','key3'].reduce(my_hash) {|m,k| m && m[k] }

Or perhaps extend Hash or just your target hash object with a nested lookup method

module NestedHashLookup
  def nest *keys
    keys.reduce(self) {|m,k| m && m[k] }
  end
end

my_hash.extend(NestedHashLookup)
my_hash.nest 'key1', 'key2', 'key3'

Oh, and how could we forget the maybe monad?

Maybe.new(my_hash)['key1']['key2']['key3']
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