gistfile1.rb

def self.last_12_monthly
  start_date = (Time.now - 11.months).beginning_of_month
  end_date   = Time.now.end_of_month
  h = Donation.calculate(:sum, :gross_amount, conditions: {
    created_at: (start_date..end_date) },
    order:      "date_trunc('month', created_at)",
    group:      ["date_trunc('month', created_at)"])
  h = h.to_a.map {|k,v| [k.to_date, v]}
  num_of_months = 0..11
  m = Hash[(num_of_months.to_a.reverse).map { |month| [month.months.ago.beginning_of_month.to_date, 0]}].merge(Hash[h])
  Hash[*m.to_a.flatten]
end