select_food_ids.rb

# Select food_ids
pets = {
  dogs: [
    {
      dog_id: '5d10ffdb-94df-4265-960c-39fa8a86e258',
      foods: [
        'e2501f63-af85-410e-8f7a-da47ffd5ce0e',
      ],
    },
    {
      dog_id: 'f80c5a13-34fa-43d2-8d64-80d0bf2e1161',
      foods: [
        '821d66ee-03e7-41dd-a1a5-248458b2b5e0',
        '5836caa8-a738-4b28-83e4-eb0e69ea82b0',
      ],
    },
  ],
  cats: [
    {
      cat_id: '76752e2c-5033-4619-a2d9-445067ccbd09',
      foods: [
        'f2944c90-ce9a-4f8e-b5cd-9a8177e918ed',
        'e2a343f8-8998-4d29-abbc-0aee27ab986f',
      ],
    },
  ],
}

food_ids = []
pets.keys.each { |key| pets[key].each { |item| food_ids << item[:foods] } }
food_ids.flatten!
puts food_ids

food_ids = []
# https://apidock.com/ruby/Hash/each_key
pets.each_key { |key| pets[key].each { |item| food_ids << item[:foods] } }
food_ids.flatten!
puts food_ids

food_ids = pets.keys.map { |key| pets[key].map { |item| item[:foods] } }
food_ids.flatten!
puts food_ids

food_ids = pets.keys.map { |key| pets[key].map { |item| item[:foods] }.flatten }.flatten
puts food_ids

# https://apidock.com/ruby/Enumerable/flat_map
food_ids = pets.keys.flat_map { |key| pets[key].flat_map { |item| item[:foods] } }
puts food_ids

# https://ruby-doc.org/core-2.7.0/Enumerator/Lazy.html
# https://blog.saeloun.com/2019/10/23/ruby-lazy-enumerators.html
food_ids = pets.keys.lazy.flat_map { |key| pets[key].flat_map { |item| item[:foods] } }.force
puts food_ids

require 'hashie'

# https://github.com/hashie/hashie#deepfind
pets.extend Hashie::Extensions::DeepFind
food_ids = pets.deep_find_all(:foods).flatten

puts food_ids