mutation_behavior.rb

original = [
  { a: { foo: 1 } },
  { b: { foo: 2 } },
  { c: { foo: 3 } },
  { d: { foo: 4 } },
  { e: { foo: 5 } },
  { f: { foo: 6 } },
  { g: { foo: 7 } },
]

copy = original.each do |item|
  item.each do |k, v|
    v[:foo] = v[:foo] + 1
  end
end

p copy
# [{:a=>{:foo=>2}}, {:b=>{:foo=>3}}, {:c=>{:foo=>4}}, {:d=>{:foo=>5}}, {:e=>{:foo=>6}}, {:f=>{:foo=>7}}, {:g=>{:foo=>8}}]
p original
# [{:a=>{:foo=>2}}, {:b=>{:foo=>3}}, {:c=>{:foo=>4}}, {:d=>{:foo=>5}}, {:e=>{:foo=>6}}, {:f=>{:foo=>7}}, {:g=>{:foo=>8}}]