✨ Include running balance when displaying transaction list

Use a SQL window function in a subquery to compute the balance, then select and sort from the subquery to get proper pagination and ordering.

lib/freedom_account/transactions.ex

@@ -15,6 +15,8 @@ defmodule FreedomAccount.Transactions do
     ],
     exports: [Transaction]
 
+  import Ecto.Query, only: [from: 1, subquery: 1]
+
   alias Ecto.Changeset
   alias FreedomAccount.Accounts.Account
   alias FreedomAccount.Error
@@ -52,12 +54,15 @@ defmodule FreedomAccount.Transactions do
   def list_fund_transactions(%Fund{} = fund, opts \\ []) do
     limit = Keyword.get(opts, :per_page, 50)
 
-    %Page{entries: transactions, metadata: metadata} =
+    fund_transactions =
       fund
       |> LineItem.by_fund()
       |> LineItem.join_transaction()
+      |> FundTransaction.with_running_balances()
+
+    %Page{entries: transactions, metadata: metadata} =
+      from(s in subquery(fund_transactions))
       |> FundTransaction.newest_first()
-      |> FundTransaction.select()
       |> Repo.paginate(
         after: opts[:next_cursor],
         before: opts[:prev_cursor],

lib/freedom_account/transactions/fund_transaction.ex

@@ -12,6 +12,7 @@ defmodule FreedomAccount.Transactions.FundTransaction do
     field :id, non_neg_integer()
     field :inserted_at, NaiveDateTime.t()
     field :memo, String.t()
+    field :running_balance, Money.t()
   end
 
   @spec compare(t(), t()) :: :eq | :gt | :lt
@@ -27,23 +28,30 @@ defmodule FreedomAccount.Transactions.FundTransaction do
   end
 
   @spec cursor_fields :: list()
-  def cursor_fields, do: [{{:transaction, :date}, :desc}, {{:line_item, :inserted_at}, :desc}, {{:line_item, :id}, :desc}]
+  def cursor_fields, do: [{:date, :desc}, {:inserted_at, :desc}, {:id, :desc}]
 
   @spec newest_first(Queryable.t()) :: Queryable.t()
   def newest_first(query) do
-    from [line_item: l, transaction: t] in query,
-      order_by: [desc: t.date, desc: l.inserted_at, desc: l.id]
+    from t in query,
+      order_by: [desc: t.date, desc: t.inserted_at, desc: t.id]
   end
 
-  @spec select(Queryable.t()) :: Queryable.t()
-  def select(query) do
+  @spec with_running_balances(Queryable.t()) :: Queryable.t()
+  def with_running_balances(query) do
     from [line_item: l, transaction: t] in query,
       select: %__MODULE__{
         amount: l.amount,
         date: t.date,
         id: l.id,
         inserted_at: l.inserted_at,
-        memo: t.memo
-      }
+        memo: t.memo,
+        running_balance: over(sum(l.amount), :fund)
+      },
+      windows: [
+        fund: [
+          order_by: [t.date, l.inserted_at, l.id],
+          partition_by: l.fund_id
+        ]
+      ]
   end
 end

lib/freedom_account_web/components/fund_transaction/index.ex

@@ -43,6 +43,9 @@ defmodule FreedomAccountWeb.FundTransaction.Index do
         <:col :let={txn} label="In">
           <span :if={Money.positive?(txn.amount)} data-role="deposit"><%= txn.amount %></span>
         </:col>
+        <:col :let={txn} label="Balance">
+          <%= txn.running_balance %>
+        </:col>
         <:empty_state>
           <div id="no-transactions">
             This fund has no transactions yet.

test/freedom_account/transactions_test.exs

@@ -151,10 +151,18 @@ defmodule FreedomAccount.TransactionsTest do
           date: transaction.date,
           id: line_item.id,
           inserted_at: line_item.inserted_at,
-          memo: transaction.memo
+          memo: transaction.memo,
+          running_balance: Money.zero(:usd)
         }
       end)
       |> Enum.sort_by(& &1, {:desc, FundTransaction})
+      |> Enum.reverse()
+      |> Enum.reduce({[], Money.zero(:usd)}, fn txn, {result, balance} ->
+        next_balance = Money.add!(balance, txn.amount)
+        txn_with_balance = %{txn | running_balance: next_balance}
+        {[txn_with_balance | result], next_balance}
+      end)
+      |> elem(0)
     end
   end
 

test/freedom_account_web/components/fund_transaction_test.exs

@@ -19,6 +19,7 @@ defmodule FreedomAccountWeb.FundTransactionTest do
       [deposit_line_item] = deposit.line_items
       withdrawal = Factory.withdrawal(account, fund)
       [withdrawal_line_item] = withdrawal.line_items
+      balance = Money.add!(deposit_line_item.amount, withdrawal_line_item.amount)
 
       conn
       |> visit(~p"/funds/#{fund}")
@@ -28,6 +29,7 @@ defmodule FreedomAccountWeb.FundTransactionTest do
       |> assert_has(table_cell(), text: "#{withdrawal.date}")
       |> assert_has(table_cell(), text: withdrawal.memo)
       |> assert_has(role("withdrawal"), text: "#{MoneyUtils.negate(withdrawal_line_item.amount)}")
+      |> assert_has(table_cell(), text: "#{balance}")
     end
 
     test "paginates transactions", %{conn: conn, fund: fund} do