rails: array of objects, how to sum values of object attributes?

Question

I have an array of objects obtained from some queries and the all the object have the attributes: id, title and quantity. The question is : How can I sum the quantity by id and save each fields? I've tried this snippet

h=Hash.new(0)
a.each do |el|
  h[el.id] += el.quantity
end

and this goes, but I lose the title! How can I have the title inside the hash h?

thanks!

UPDATE to a solution:

I thought that the data for the a array could be many thousands of records, so I decided to anticipate the aggregation to sum the quantity. Thank's to a example of solution of , now each time I get the block of records from master table, I elaborate the data in this way

@agg = Hash.new(Array.new)
master.scope.each do |mst|
  recs = mst.select.........
  recs.each do |el|
      store = @agg[el.id].empty? ? 0 : @agg[el.id][1]
      @agg[el.id] = [el.title, el.quantity+store]
  end
end

at the end, @agg holds all and only the aggregated data.

If someone can provide a more general or elegant solution come forward.

Thank's to all.

Giorgio

Sebastian Iorga · Accepted Answer · 2013-09-11 11:01:20Z

1

A hash element has 2 values in it, the key and the, well, value; hence h[key]=value.

For your goal you can just make the value be an array/hash, the first element of which is the title and the second element the quantity.

However what you are doing is not summing/accumulating anything. You are just building a hash out of the elements. Every time you iterate you create h[id] = h[quantity] for that specific id. You're just turning your el objects into hashes. If you want to accumulate the values do this.

sum = 0
h = Hash.new(Array.new) 
a.each do |el|
  sum += el.quantity
  h[el.id] = [el.title, sum]
end

If what you ultimately want is just to end up with your objects accumulating quantities in order of ids try:

prior = nil
a.each do |el|
  el.quantity += prior.quantity unless prior.nil?
  prior = el
end

answered Sep 11, 2013 at 11:01

Sebastian Iorga

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3 Comments

gbonline Over a year ago

Thank's, but i think I may have been to brief on my description above.

gbonline Over a year ago

I need to sum quantity for each id and keep title to the left for print purpose. In sql could be select id, max(title), sum(quantity) from table group by id. I would like to make this work on sql, but i haven't find a way to aggregate ActiveRecord::Relation objects

gbonline Over a year ago

Thank's to your example i've made a solution to my problem, See my edited question

Miotsu · Accepted Answer · 2013-09-11 15:41:21Z

0

A hash is a collection of key, value pairs, so you can't store id, quantity, and title in a single hash element. What you want is a hash with id or title as the key and a nested hash with title or id and quantity as a value. Something like:

h = Hash.new { |hash, key| hash[key] = {} }
a.each do |el|
  if h[el.id][el.title].nil?
    h[el.id][el.title] = el.quantity
  else 
    h[el.id][el.title] += el.quantity
  end
end

edited Sep 11, 2013 at 15:41

answered Sep 11, 2013 at 10:52

Miotsu

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4 Comments

Miotsu Over a year ago

ah, now I see what you meant

gbonline Over a year ago

thank's, this is very elegant, but tried right now, in rails 3.2.13 console and get error: Undefined method + for nil class

Miotsu Over a year ago

of course! when there's no quantity to add to... new edit coming.

gbonline Over a year ago

Thanks this works! I tried it now, and it's a bit along the way to obtain the value of quantity : h[key].values[0].

tihom · Accepted Answer · 2013-09-11 18:09:36Z

0

if the title is unique to id you can do this,

h=Hash.new
a.each do |el|
  h[el.id] ||= {:title => el.title, :quantity => 0}
  h[el.id][:quantity] += el.quantity
end

Note that this would keep the first value of title it encounters for a id.

It might be faster if you can do it within the db query. Sthing like

recs = mst.select("SUM(quantity) as total_quantity, id, title").group(:id)

This would return the records grouped by unique id and the quantity for each group is summed and is available as rec.total_quantity.

edited Sep 11, 2013 at 18:09

answered Sep 11, 2013 at 10:53

tihom

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3 Comments

gbonline Over a year ago

thank's, but tried right now it get error: Can't convert Symbol into integer

tihom Over a year ago

fixed the issue by removing the 0 passed to initiate the hash. It was making the default value to be zero for all keys. Not needed as we initiate quantity as 0 within the loop.

gbonline Over a year ago

This solution is truly elegant. for your updated answer: my mst recordset contains only rows with differents id, so gruping by id don't change anithing. I need to repeat the cycle because recs get values from two model: from mst a get parameters to filter the second. For brevity I've omitted the complete query. many thanks

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