ruby 50 lines · 7 steps

Memoized report metrics in Ruby in Rails

A report object computes each metric once and caches it, so a single summary call reuses shared queries.

Explained by highlit
Rails Intermediate memoization query-composition service-object caching aggregation
1class ReportGenerator
2 def initialize(account, period)
3 @account = account
4 @period = period
5 end
6 
7 def summary
8 {
9 revenue: total_revenue,
10 churn_rate: churn_rate,
11 active_customers: active_customers.size,
12 top_products: top_products
13 }
14 end
15 
16 def total_revenue
17 @total_revenue ||= paid_invoices.sum(&:amount_cents) / 100.0
18 end
19 
20 def churn_rate
21 return @churn_rate if defined?(@churn_rate)
22 
23 starting = @account.customers.active_at(@period.begin).count
24 @churn_rate = starting.zero? ? 0.0 : (cancelled_count.to_f / starting).round(4)
25 end
26 
27 def active_customers
28 @active_customers ||= @account.customers.active_during(@period).to_a
29 end
30 
31 private
32 
33 def paid_invoices
34 @paid_invoices ||= @account.invoices.paid.where(issued_at: @period)
35 end
36 
37 def cancelled_count
38 @cancelled_count ||= @account.subscriptions.cancelled_during(@period).count
39 end
40 
41 def top_products
42 @top_products ||= paid_invoices
43 .flat_map(&:line_items)
44 .group_by(&:product_id)
45 .transform_values { |items| items.sum(&:amount_cents) }
46 .sort_by { |_, total| -total }
47 .first(5)
48 .map(&:first)
49 end
50end
01 / 01
STEP 01

Walkthrough

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Three takeaways
  1. 1Memoizing each metric method turns repeated calls into cheap lookups after the first computation.
  2. 2Use defined? instead of ||= when a legitimate result can be falsey like 0.0.
  3. 3Grouping metric logic into one object lets several methods share the same underlying query cheaply.

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