A array of hashes summing method

gistfile1.rb

  def sum_by(arr_hash, value_key, group_key=nil, &block)
    if group_key.instance_of? Proc
      block = group_key
      group_key = nil
    end
    arr_hash.inject({}) do |sums, row|
      group_key = (block_given?) ? block.call(row) : group_key
      sums[group_key] ||= 0.0
      sums[group_key] += row[value_key] || 0.0
      sums
    end
  end