Basic Account Balance Sync Algorithm (#501)

* Sketch out sync interface

* Add basic account sync algorithm

* Update logic for final balance in series

* Remove start_date concept

* Clean up tests

* Improve clarity of test

* Update app/models/account.rb

Co-authored-by: Rob Zolkos <rob@zolkos.com>
Signed-off-by: Zach Gollwitzer <zach.gollwitzer@gmail.com>

* Update app/models/transaction.rb

Co-authored-by: Rob Zolkos <rob@zolkos.com>
Signed-off-by: Zach Gollwitzer <zach.gollwitzer@gmail.com>

* Update app/models/valuation.rb

Co-authored-by: Rob Zolkos <rob@zolkos.com>
Signed-off-by: Zach Gollwitzer <zach.gollwitzer@gmail.com>

* Re-organize code, simplify job interface

* Consolidate balance calculations

* More cleanup

---------

Signed-off-by: Zach Gollwitzer <zach.gollwitzer@gmail.com>
Co-authored-by: Rob Zolkos <rob@zolkos.com>

app/models/account/balance_calculator.rb

@@ -0,0 +1,38 @@
+class Account::BalanceCalculator
+    def initialize(account)
+      @account = account
+    end
+
+    def daily_balances(start_date = nil)
+      calc_start_date = [ start_date, @account.effective_start_date ].compact.max
+
+      valuations = @account.valuations.where("date >= ?", calc_start_date).order(:date).select(:date, :value)
+      transactions = @account.transactions.where("date > ?", calc_start_date).order(:date).select(:date, :amount)
+      oldest_entry = [ valuations.first, transactions.first ].compact.min_by(&:date)
+
+      net_transaction_flows = transactions.sum(&:amount)
+      implied_start_balance = oldest_entry.is_a?(Valuation) ? oldest_entry.value : @account.balance + net_transaction_flows
+
+      prior_balance = implied_start_balance
+      calculated_balances = ((calc_start_date + 1.day)...Date.current).map do |date|
+        valuation = valuations.find { |v| v.date == date }
+
+        if valuation
+            current_balance = valuation.value
+        else
+            current_day_net_transaction_flows = transactions.select { |t| t.date == date }.sum(&:amount)
+            current_balance = prior_balance - current_day_net_transaction_flows
+        end
+
+        prior_balance = current_balance
+
+        { date: date, balance: current_balance, updated_at: Time.current }
+      end
+
+      [
+        { date: calc_start_date, balance: implied_start_balance, updated_at: Time.current },
+        *calculated_balances,
+        { date: Date.current, balance: @account.balance, updated_at: Time.current } # Last balance must always match "source of truth"
+      ]
+    end
+end

app/models/account/syncable.rb

@@ -0,0 +1,26 @@
+module Account::Syncable
+    extend ActiveSupport::Concern
+
+    def sync_later
+        AccountSyncJob.perform_later self
+    end
+
+    def sync
+        update!(status: "SYNCING")
+        synced_daily_balances = Account::BalanceCalculator.new(self).daily_balances
+        self.balances.upsert_all(synced_daily_balances, unique_by: :index_account_balances_on_account_id_and_date)
+        self.balances.where("date < ?", effective_start_date).delete_all
+        update!(status: "OK")
+    rescue => e
+        update!(status: "ERROR")
+        Rails.logger.error("Failed to sync account #{id}: #{e.message}")
+    end
+
+    # The earliest date we can calculate a balance for
+    def effective_start_date
+        first_valuation_date = self.valuations.order(:date).pluck(:date).first
+        first_transaction_date = self.transactions.order(:date).pluck(:date).first
+
+        [ first_valuation_date, first_transaction_date&.prev_day ].compact.min || Date.current
+    end
+end