Improve efficiency for a map command? - ruby-on-rails

Question

I have 4 databases in my rails program, Products, Offerings, OfferingProducts, and Orders
Sample Offering: Medium Kit
Medium Kit Contains 2 Products (This is set up in the OfferingProduct db)
1 Small
2 Medium
customer Orders 2 medium kits
this program should account for 2 Small and 4 Medium from this order and many orders similar
I'm trying to set up an inventory system to track how many of each product are sold every week so that I can reorder products. To calculate this I have:
def get_trend
product = Product.first
customer_orders =[]
product.offerings.each do |o|
customer_orders = customer_orders + o.orders
end
o=customer_orders.group_by { |t| t.date.beginning_of_week }
y = []
op=self.offering_products;
o.sort.each do |week, orders|
y << orders.map { |o| o.quantity*op.find_by_offering_id(o.offering).quantity }.sum
end
return y
end
This seems to work, but it takes about 20 seconds to calculate. Is there any way to make this faster? I know most of the time is taken by the
y << orders.map { |o| o.quantity*op.find_by_offering_id(o.offering).quantity }.sum
line which calculates the number of products ordered in a given week. Any thoughts?

Answer 1

The problem is you're using iterations (i.e loops in application code) when you'd be better off using declarations (i.e. joins in the database). Loops will never be as fast as joins, and moving the dataset into the application's memory won't be helping either.
I'm guessing you're using ActiveRecord? If so, this may help with how to query with joins: http://guides.rubyonrails.org/active_record_querying.html

Answer 2

Here is what I ended up doing thanks to the info from Iain
def get_trend
sql = ActiveRecord::Base.connection()
if Rails.env == "production"
d=sql.execute("SELECT SUM(orders.quantity * offering_products.quantity), EXTRACT(ISOYEAR FROM orders.date) AS year, EXTRACT(WEEK FROM orders.date) AS week " +
" FROM orders " +
" INNER JOIN offerings ON offerings.id = orders.offering_id " +
" INNER JOIN offering_products ON offering_products.offering_id = offerings.id " +
" INNER JOIN products ON products.id = offering_products.product_id " +
" WHERE (products.id = #{self.id}) GROUP BY year, week ORDER BY year, week ")
y=d.map { |a| a["sum"].to_i }
else
d=sql.execute("SELECT SUM(orders.quantity * offering_products.quantity), strftime('%G-%V', orders.date) AS year " +
" FROM orders " +
" INNER JOIN offerings ON offerings.id = orders.offering_id " +
" INNER JOIN offering_products ON offering_products.offering_id = offerings.id " +
" INNER JOIN products ON products.id = offering_products.product_id " +
" WHERE (products.id = #{self.id}) GROUP BY year")
y=d.map { |a| a[0] }
end
return y
end
Update: needed to differentiate the code between production and local, because heroku uses postgresql, and the date functions work differently:( which also means the result isnt exactly the same, because the functions treat the first week of the year differently.