Merge two hashes together and performing computation

Question

I am attempting to combine two hashes together, and am having trouble figuring out the best method to do it. I feel like there must be an easy way to do it with enumerables...

I'd like to turn this:

[{ id: 5, count: 10 }, { id: 6, count: -3 }, { id: 5, count: -2 }, { id: 3, count: 4}]

into this:

[{ id: 5, count: 8 }, { id: 6, count: -3 }, { id: 3, count: 4}]

So that hashes with the same "id" are summed together. Anyone have any ideas on how to quickly do this?

I tried Hash.merge but that didn't work correctly...

Answer 1

Here is a way :

hash = [{ id: 5, count: 10 }, { id: 6, count: -3 }, { id: 5, count: -2 }, { id: 3, count: 4}]
merged_hash = hash.group_by { |h| h[:id] }.map do |_,v|
  v.reduce do |h1,h2| 
    h1.merge(h2) { |k,o,n| k == :count ? o + n : o }
  end
end

merged_hash 
# => [{:id=>5, :count=>8}, {:id=>6, :count=>-3}, {:id=>3, :count=>4}]

Look at these methods Hash#merge, Enumerable#reduce and Enumerable#group_by.