Ruby Style: How to check whether a nested hash element exists

Question

Consider a "person" stored in a hash. Two examples are:

fred = {:person => {:name => "Fred", :spouse => "Wilma", :children => {:child => {:name => "Pebbles"}}}}
slate = {:person => {:name => "Mr. Slate", :spouse => "Mrs. Slate"}} 

If the "person" doesn't have any children, the "children" element is not present. So, for Mr. Slate, we can check whether he has parents:

slate_has_children = !slate[:person][:children].nil?

So, what if we don't know that "slate" is a "person" hash? Consider:

dino = {:pet => {:name => "Dino"}}

We can't easily check for children any longer:

dino_has_children = !dino[:person][:children].nil?
NoMethodError: undefined method `[]' for nil:NilClass

So, how would you check the structure of a hash, especially if it is nested deeply (even deeper than the examples provided here)? Maybe a better question is: What's the "Ruby way" to do this?

Answer 1

The most obvious way to do this is to simply check each step of the way:

has_children = slate[:person] && slate[:person][:children]

Use of .nil? is really only required when you use false as a placeholder value, and in practice this is rare. Generally you can simply test it exists.

Update: If you're using Ruby 2.3 or later there's a built-in dig method that does what's described in this answer.

If not, you can also define your own Hash "dig" method which can simplify this substantially:

class Hash
  def dig(*path)
    path.inject(self) do |location, key|
      location.respond_to?(:keys) ? location[key] : nil
    end
  end
end

This method will check each step of the way and avoid tripping up on calls to nil. For shallow structures the utility is somewhat limited, but for deeply nested structures I find it's invaluable:

has_children = slate.dig(:person, :children)

You might also make this more robust, for example, testing if the :children entry is actually populated:

children = slate.dig(:person, :children)
has_children = children && !children.empty?

Answer 2

With Ruby 2.3 we'll have support for the safe navigation operator: https://www.ruby-lang.org/en/news/2015/11/11/ruby-2-3-0-preview1-released/

has_children now could be written as:

has_children = slate[:person]&.[](:children)

dig is being added as well:

has_children = slate.dig(:person, :children)

Answer 3

Another alternative:

dino.fetch(:person, {})[:children]

Answer 4

You can use the andand gem:

require 'andand'

fred[:person].andand[:children].nil? #=> false
dino[:person].andand[:children].nil? #=> true

You can find further explanations at http://andand.rubyforge.org/.

Answer 5

One could use hash with default value of {} - empty hash. For example,

dino = Hash.new({})
dino[:pet] = {:name => "Dino"}
dino_has_children = !dino[:person][:children].nil? #=> false

That works with already created Hash as well:

dino = {:pet=>{:name=>"Dino"}}
dino.default = {}
dino_has_children = !dino[:person][:children].nil? #=> false

Or you can define [] method for nil class

class NilClass
  def [](* args)
     nil
   end
end

nil[:a] #=> nil

Answer 6

Traditionally, you really had to do something like this:

structure[:a] && structure[:a][:b]

However, Ruby 2.3 added a feature that makes this way more graceful:

structure.dig :a, :b # nil if it misses anywhere along the way

There is a gem called ruby_dig that will back-patch this for you.

Answer 7

def flatten_hash(hash)
  hash.each_with_object({}) do |(k, v), h|
    if v.is_a? Hash
      flatten_hash(v).map do |h_k, h_v|
        h["#{k}_#{h_k}"] = h_v
      end
    else
      h[k] = v
    end
  end
end

irb(main):012:0> fred = {:person => {:name => "Fred", :spouse => "Wilma", :children => {:child => {:name => "Pebbles"}}}}
=> {:person=>{:name=>"Fred", :spouse=>"Wilma", :children=>{:child=>{:name=>"Pebbles"}}}}

irb(main):013:0> slate = {:person => {:name => "Mr. Slate", :spouse => "Mrs. Slate"}}
=> {:person=>{:name=>"Mr. Slate", :spouse=>"Mrs. Slate"}}

irb(main):014:0> flatten_hash(fred).keys.any? { |k| k.include?("children") }
=> true

irb(main):015:0> flatten_hash(slate).keys.any? { |k| k.include?("children") }
=> false

This will flatten all the hashes into one and then any? returns true if any key matching the substring "children" exist. This might also help.

Answer 8

dino_has_children = !dino.fetch(person, {})[:children].nil?

Note that in rails you can also do:

dino_has_children = !dino[person].try(:[], :children).nil?   # 

Answer 9

Here is a way you can do a deep check for any falsy values in the hash and any nested hashes without monkey patching the Ruby Hash class (PLEASE don't monkey patch on the Ruby classes, such is something you should not do, EVER).

(Assuming Rails, although you could easily modify this to work outside of Rails)

def deep_all_present?(hash)
  fail ArgumentError, 'deep_all_present? only accepts Hashes' unless hash.is_a? Hash

  hash.each do |key, value|
    return false if key.blank? || value.blank?
    return deep_all_present?(value) if value.is_a? Hash
  end

  true
end

Answer 10

Simplifying the above answers here:

Create a Recursive Hash method whose value cannot be nil, like as follows.

def recursive_hash
  Hash.new {|key, value| key[value] = recursive_hash}
end

> slate = recursive_hash 
> slate[:person][:name] = "Mr. Slate"
> slate[:person][:spouse] = "Mrs. Slate"

> slate
=> {:person=>{:name=>"Mr. Slate", :spouse=>"Mrs. Slate"}}
slate[:person][:state][:city]
=> {}

If you don't mind creating empty hashes if the value does not exists for the key :)

Answer 11

Given

x = {:a => {:b => 'c'}}
y = {}

you could check x and y like this:

(x[:a] || {})[:b] # 'c'
(y[:a] || {})[:b] # nil

Answer 12

You can try to play with

dino.default = {}

Or for example:

empty_hash = {}
empty_hash.default = empty_hash

dino.default = empty_hash

That way you can call

empty_hash[:a][:b][:c][:d][:e] # and so on...
dino[:person][:children] # at worst it returns {}

Answer 13

Thks @tadman for the answer.

For those who want perfs (and are stuck with ruby < 2.3), this method is 2.5x faster:

unless Hash.method_defined? :dig
  class Hash
    def dig(*path)
      val, index, len = self, 0, path.length
      index += 1 while(index < len && val = val[path[index]])
      val
    end
  end
end

and if you use RubyInline, this method is 16x faster:

unless Hash.method_defined? :dig
  require 'inline'

  class Hash
    inline do |builder|
      builder.c_raw '
      VALUE dig(int argc, VALUE *argv, VALUE self) {
        rb_check_arity(argc, 1, UNLIMITED_ARGUMENTS);
        self = rb_hash_aref(self, *argv);
        if (NIL_P(self) || !--argc) return self;
        ++argv;
        return dig(argc, argv, self);
      }'
    end
  end
end

Answer 14

You can also define a module to alias the brackets methods and use the Ruby syntax to read/write nested elements.

UPDATE: Instead of overriding the bracket accessors, request Hash instance to extend the module.

module Nesty
  def []=(*keys,value)
    key = keys.pop
    if keys.empty? 
      super(key, value) 
    else
      if self[*keys].is_a? Hash
        self[*keys][key] = value
      else
        self[*keys] = { key => value}
      end
    end
  end

  def [](*keys)
    self.dig(*keys)
  end
end

class Hash
  def nesty
    self.extend Nesty
    self
  end
end

Then you can do:

irb> a = {}.nesty
=> {}
irb> a[:a, :b, :c] = "value"
=> "value"
irb> a
=> {:a=>{:b=>{:c=>"value"}}}
irb> a[:a,:b,:c]
=> "value"
irb> a[:a,:b]
=> {:c=>"value"}
irb> a[:a,:d] = "another value"
=> "another value"
irb> a
=> {:a=>{:b=>{:c=>"value"}, :d=>"another value"}}

Answer 15

I don't know how "Ruby" it is(!), but the KeyDial gem which I wrote lets you do this basically without changing your original syntax:

has_kids = !dino[:person][:children].nil?

becomes:

has_kids = !dino.dial[:person][:children].call.nil?

This uses some trickery to intermediate the key access calls. At call, it will try to dig the previous keys on dino, and if it hits an error (as it will), returns nil. nil? then of course returns true.

Answer 16

You can use a combination of & and key? it is O(1) compared to dig which is O(n) and this will make sure person is accessed without NoMethodError: undefined method `[]' for nil:NilClass

fred[:person]&.key?(:children) //=>true
slate[:person]&.key?(:children)