Ruby CSV library is smart and will select just keys in the headers if given headers and a hash

main.rb

require 'csv'
filename = "/tmp/csv_test.csv"

things_to_write = [
  {a: "a", b: "b", c: "c"},
  {c: "c", b: "b", a: "a", does_not: "exist"},
  {a: "a", c: "c"}
]
CSV.open(filename, "wb", headers: things_to_write.first.keys) do |csv|
  things_to_write.each do |thing|
    csv << thing
  end
end

`cat #{filename}`