Save the most recent months of a hash - ruby-on-rails

Question

I have an array of hashes. Something like this...
transactions = [{"date"=>"2014-07-21", "amount"=>200},
{"date"=>"2012-06-21", "amount"=>400},
{"date"=>"2014-08-21", "amount"=>100},
{"date"=>"2014-08-12", "amount"=>150},
{"date"=>"2014-06-15", "amount"=>230}
{"date"=>"2013-05-21", "amount"=>900},]
I want to be able to save each months total amounts and then show the most recent 3 months to todays date and their total amount. Something like this...
Totals: 06-14 $230
07-14 $200
08-14 $250
I have this method but i am not sure how to get only the last 3 months to put in my database field and how to print it out.
def income_by_month
#payroll_transactions = current_user.transactions
#recent_payroll = #payroll_transactions.find_all {90.days.ago.to_date..Date.today}.map #finds transactions within 90 days
#amount_by_month = #recent_payroll.group_by { |t| t.date.to_date.month }.map do |month, transactions|
[month, transactions.sum(:amount)] #Groups transactions by month and adds month total
end.to_h
-EDIT-
I figured out a method to only get the transactions from the last 30 days I updated my method to show it. Now my question is how do I save the answer (Do i save it in one field as an Array?) and then how to show the answer in my view. Like I show it here. How do I print each key and value line by line in an order?
Totals: 06-14 $230
07-14 $200
08-14 $250
-EDIT-
Sorry my database is a mongoid db. And I want to save the most recent 3 months to todays date regardless of if an amount is available.

Answer 1

Let me start with a quick note on your code snippet:
group_by { |t| t.date.to_date.month }
Note that grouping objects by a single month does not take a year in count, so it would end summing up amounts for transactions of both 2012 and 2014 years in a one container. So what you really want is to group based on both month and year values.
Thinking of reducing the amount of redundant iterations through the input array (and using unnecessary aggregations), I've came to the following suggestion:
last_months = transactions.map{|i| Date.parse(i["date"]).strftime("%m-%Y")}.uniq.sort.last(3)
result = last_months.inject({}){|result, input| result[input] = 0; result}
transactions.inject(result) do |result, object|
# NOTE: we're already doing dates parsing and strftime two times here.
# In case you operate on Date objects themselves in your code, this is not the case.
# But the real perfomance measurement between summing all values up
# and strftiming more than once should be done additionally.
month = Date.parse(object["date"]).strftime("%m-%Y")
result[month] += object["amount"] if result[month]
result
end
# result now equals to {"06-2014"=>230, "07-2014"=>200, "08-2014"=>250}
First, we obtain those last three months (and years).
Next we create a hash to contain aggregated values with only those last months keys. At the end we sum up amount for only those transactions which seem to be one of the latter 3 months.
So, as long as ruby hashes (ruby v.1.9+) preserve the keys order, you can simply iterate over them to print out:
result.each{|k,v| puts "#{k}: #{v}"}
# 06-2014: 230
# 07-2014: 200
# 08-2014: 250
One last thing to note here is that doing this kind of aggregation inside of your server code is not quite efficient. Much more tempting option would be to move this calculations to your database layer.

Answer 2

ActiveSupport has some pretty slick date methods like Date#beginning_of_month:
require "date"
require "active_support/core_ext"
def process_transaction_group(month, transactions)
{
month: month.strftime("%Y/%m"),
total: transactions.map {|t| t["amount"] }.reduce(:+)
}
end
def process_transactions(transactions)
transactions
.group_by {|t| Date.parse(t["date"]).beginning_of_month }
.select {|month, _trxs| month < 3.months.ago }
.map {|month, trxs| process_transaction_group(month, trxs) }
end
###############
transactions = [{"date"=>"2014-07-21", "amount"=>200},
{"date"=>"2012-06-21", "amount"=>400},
{"date"=>"2014-08-21", "amount"=>100},
{"date"=>"2014-08-12", "amount"=>150},
{"date"=>"2014-06-15", "amount"=>230},
{"date"=>"2013-05-21", "amount"=>900}]
process_transactions(transactions)
#=> [{:month=>"2014/07", :total=>200}, {:month=>"2012/06", :total=>400}, {:month=>"2014/08", :total=>250}, {:month=>"2014/06", :total=>230}, {:month=>"2013/05", :total=>900}]