add initial time series function to ProductBalance

app/models/product_balance.rb

@@ -12,4 +12,21 @@ def self.by_account
       hsh[product_balance.account_id] = product_balance.amount_cents
     end
   end
+
+  def self.time_series(date_range = 1.month.ago.to_date..Date.current.to_date)
+    by_date = by_date(date_range)
+    date_range.each_with_object({}) do |date, hash|
+      hash[date] = by_date[date] || hash[date - 1] || starting_balance(as_of: date_range.first)
+    end
+  end
+
+  def self.by_date(date_range)
+    where(date: date_range).each_with_object({}) do |balance, hash|
+      hash[balance.date] = balance.amount_cents
+    end
+  end
+
+  def self.starting_balance(as_of: Date.current)
+    where('date < ?', as_of).order(date: :asc).last.try(:amount_cents) || 0
+  end
 end

app/views/accounts/index.html.haml

@@ -39,4 +39,5 @@
       %h4
         Daily Balance
         %small Accounts Receivable for Product 4 (in cents)
-      = debug Account.daily_balance(:accounts_receivable, for_product: 4, date_range: Date.new(2014, 12, 25)..Date.new(2015, 1, 15))
+      / = debug Account.daily_balance(:accounts_receivable, for_product: 4, date_range: Date.new(2014, 12, 25)..Date.new(2015, 1, 15))
+      = debug ProductBalance.for_product(4).where(account: Account.accounts_receivable).time_series(Date.new(2014, 12, 25)..Date.new(2015, 1, 15))