Ruby on Rails: Create an Array of Sums from Array of Objects

Question

I have an array of objects that looks like this:

[#<Holding id: 5, user_id: 21, outcome_id: 7, quantity: 200, cost_basis: nil, created_at: "2011-12-15 20:52:05", updated_at: "2011-12-15 20:52:05">,
 #<Holding id: 6, user_id: 21, outcome_id: 7, quantity: 750, cost_basis: nil, created_at: "2011-12-15 21:31:30", updated_at: "2011-12-15 21:31:30">,
 #<Holding id: 7, user_id: 21, outcome_id: 9, quantity: 1231, cost_basis: nil, created_at: "2011-12-15 21:35:31", updated_at: "2011-12-15 21:35:31">]

I retrieved this by using my model's find_all_by_user_id (Holding.find_all_by_user_id(21)). What I need, however, is an array of sums. So, for instance, what I'd like to get instead in this instance is:

[#<Holding id: 5, user_id: 21, outcome_id: 7, quantity: 950, cost_basis: nil, created_at: "2011-12-15 20:52:05", updated_at: "2011-12-15 20:52:05">,
     #<Holding id: 7, user_id: 21, outcome_id: 9, quantity: 1231, cost_basis: nil, created_at: "2011-12-15 21:35:31", updated_at: "2011-12-15 21:35:31">]

What would make it even better is if I could pull from my Outcomes model the Outcome.description and have it in this second array along with the id. Do I just have to loop over and create sums where outcome_id's match or is there a better way?

the Tin Man · Accepted Answer · 2011-12-16 00:13:33Z

2

Personally, I would do this in SQL - SUM(quantity) and GROUP BY outcome_id.

If you want it in Ruby, you can do

holdings = Holding.find_all_by_user_id(21)

sums = Hash.new(0)

holdings.each do |h|
  sums[h.outcome_id] += h.quantity
end

sums is now a hash with key being all the outcome_ids and the value being the total quantity.

Please note that in both solutions (SQL and Ruby) you will lose the information in the other fields (holding_id, user_id, cost_basis, timestamps), which may not be what you want.

edited Dec 16, 2011 at 0:13

the Tin Man

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answered Dec 15, 2011 at 23:33

Nils Landt

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1 Comment

the Tin Man Over a year ago

I agree doing it in SQL is the better solution. It returns a lot less data which is always helpful, plus the DBM is made for that sort of lifting. The only time I'd retrieve the entire row is if I have to do subsequent processing of the data based on the sums.

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