Use an Array to access nested Hash keys

Question

I have an Array of strings which represent a path of nested hash keys e.g.

["foo", "bar", "baz"]

I want to create a function which takes in the Array and a value and sets the value for the key provided in the Array, so when I call it with the example, it sets the value for

Hash["foo"]["bar"]["baz"].

Is there a method that can do that. Is there a way to chain the elements of an Array into calls for hash keys with #inject ?

Any help is highly appreciated.

To specify the question:

I have the following code:

def anonymize_data_hash(data, path=[])
  if data.is_a?(Hash)
    data.each do |key, value|
      anonymize_data_hash(value, path + [key])
    end
  elsif data.is_a?(Array)
    data.each do |value|
      anonymize_data_hash(value, path)
    end
  else
    path = path.clone
    key = path.shift
    path = (path + [data]).map(&:to_s)

    send("#{key}")[path] = "xxxxxx"
    save
  end
end

the anonymize_data_hash method sends an method(attribute) call to a mode which is a serialized hash. Path is an array of strings. In order make the above function work I need to turn the array of string into a nested Hash call.

The Hash already exists I need to access it with the values given in the Array. Thank You for Your help.

Simple Lime · Accepted Answer · 2018-08-09 18:29:43Z

9

You could make use of the newer method Hash#dig (ruby 2.3+) to access the nested hash, and then set the value on that:

# ideally this might be a method on Hash, so you wouldn't need to pass it in
def deep_set(hash, path, value)
  *path, final_key = path
  to_set = path.empty? ? hash : hash.dig(*path)

  return unless to_set
  to_set[final_key] = value
end

hash = {
  "foo" => {
    "bar" => { }
  }
}

deep_set(hash, ["foo", "bar", "baz"], "xxxxxx")
deep_set(hash, ["baz"], "yyyyyy")
deep_set(hash, ["foo", "baz", "bar"], "zzzzzz")
puts hash
# => {"foo"=>{"bar"=>{"baz"=>"xxxxxx"}}, "baz"=>"yyyyyy"}

answered Aug 9, 2018 at 18:29

Simple Lime

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8 Comments

Sebastian Peter Over a year ago

Looks very nice. I will try it and accept it when it works in my rails app.

Simple Lime Over a year ago

You need to deconstruct the array, hash.dig(*path), notice the asterisk * that turns the array into an argument list that works for dig

Sebastian Peter Over a year ago

So in my example I would retrieve the hash with send("#{key}") and save it in a locale variable an then call your deep_set method on it.

Simple Lime Over a year ago

yeah if send("#{key}") returns the hash object (which btw, could just be send(key), unless key might not be a string or symbol), then you would call it with deep_set(send(key), path, "xxxxxx"), or you could, of course, save it into a local variable and pass that into the method, but it's not needed

Sebastian Peter Over a year ago

Yeah thats true. Another thing I am adding is that I will only set the key if it is present. The serialized hash I am anonymizing is submitted form_data. So I don't want create and anonymize keys which aren't present.

|

wteuber · Accepted Answer · 2018-08-10 13:35:32Z

Simple Lime's solution requires a well-formatted hash to begin with. If addition to the hash depends on that, you highly depend on the input data and its order to determine if a value is added or not (e.g. "zzzzzz" in the example). You might want to (re-)define the hash's default_proc to create the structure of the hash on the fly. Here is an example:

def nest(array, value)
  hash = Hash.new { |h, k| h[k] = Hash.new(&h.default_proc) }
  hash.dig(*array[0..-2])[array.fetch(-1)] = value
  hash
end

nest ["foo", "bar", "baz"], :test
# => {"foo"=>{"bar"=>{"baz"=>:test}}}

Since the hash already exists, I suggest proper initialisation of its default_proc.

hash = Hash.new { |h, k| h[k] = Hash.new(&h.default_proc) }

If that's not possible, you can still overwrite it later as well

hash = {}  # unchange initialisation 

hash.default_proc = -> (h, k) { h[k] = Hash.new(&h.default_proc) }

def nest(hash, array, value)
  hash.dig(*array[0..-2])[array.fetch(-1)] = value
  hash
end

nest hash, ["foo", "bar", "baz"], :test
# => {"foo"=>{"bar"=>{"baz"=>:test}}}

I hope you find this helpful too.

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