Find hash in another array and substract in Ruby -

i have 2 arrays of hashes this:

a = [ { car_id: 1, motor_id: 1, quantity: 5 },       { car_id: 1, motor_id: 2, quantity: 6 },       { car_id: 5, motor_id: 3, quantity: 3 } ]  b = [ { car_id: 1, motor_id: 1, quantity: 2 },       { car_id: 1, motor_id: 2, quantity: 3 } ] 

i want substract quantities each hash in b hashes in a hashes have same car_id & motor_id. so, expected result be:

c = [ {car_id: 1, motor_id: 1, quantity: 3},       {car_id: 1, motor_id: 2, quantity: 3},       {car_id: 5, motor_id: 3, quantity: 3 } ] 

what way in ruby? thoughts iterate on a, , each element find if there in b have same car_id , motor_id, if so, substract, , continue.

i suggest first create hash bqty b that, each element (hash) g of b, maps [g[:car_id], g[:motor_id]] g[:quantity]:

bqty = b.each_with_object({}) {|g,h| h[[g[:car_id], g[:motor_id]]] = g[:quantity]}   #=> {[1, 1]=>2, [1, 2]=>3} 

next, map each element (hash) g of a desired hash. done merging g empty hash h (or g.dup), then, if there element of bqty key key = [h[:car_id], h[:motor_id]], subtract bqty[key] h[:quantity]. note leaves a , b unchanged.

a.map |g|   {}.merge(g).tap |h|     key = [h[:car_id], h[:motor_id]]     h[:quantity] -= bqty[key] if bqty.key?(key)   end end   #=> [{:car_id=>1, :motor_id=>1, :quantity=>3},   #    {:car_id=>1, :motor_id=>2, :quantity=>3},   #    {:car_id=>5, :motor_id=>3, :quantity=>3}] 

an alternative antepenultimate¹ line is:

    h[:quantity] -= bqty[key].to_i 

since nil.to_i #=> 0.

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