improvement: add money sub & negation operators

lib/ash_money/ash_postgres_extension.ex

@@ -3,16 +3,27 @@ if Code.ensure_loaded?(AshPostgres.CustomExtension) do
     @moduledoc """
     Installs the `money_with_currency` type and operators/functions for Postgres.
     """
-    use AshPostgres.CustomExtension, name: :ash_money, latest_version: 2
+    use AshPostgres.CustomExtension, name: :ash_money, latest_version: 3
+
+    def install(2) do
+      """
+      #{Money.DDL.execute_each(add_money_sub())}
+      #{Money.DDL.execute_each(add_money_neg())}
+      """
+    end
 
     def install(1) do
       """
+      #{Money.DDL.execute_each(add_money_sub())}
       #{Money.DDL.execute_each(add_money_mult())}
+      #{Money.DDL.execute_each(add_money_neg())}
       """
     end
 
     def install(0) do
       """
+      #{Money.DDL.execute_each(add_money_sub())}
+      #{Money.DDL.execute_each(add_money_neg())}
       #{Money.DDL.execute_each(Money.DDL.create_money_with_currency())}
       #{Money.DDL.execute_each(Money.DDL.define_plus_operator())}
       #{Money.DDL.execute_each(Money.DDL.define_minmax_functions())}
@@ -21,6 +32,14 @@ if Code.ensure_loaded?(AshPostgres.CustomExtension) do
       """
     end
 
+    def uninstall(3) do
+      """
+      #{Money.DDL.execute_each(remove_money_sub())}
+      #{Money.DDL.execute_each(remove_money_neg())}
+      #{uninstall(2)}
+      """
+    end
+
     def uninstall(v) when v in [0, 1, 2] do
       """
       #{Money.DDL.execute_each(remove_money_mult())}
@@ -31,6 +50,81 @@ if Code.ensure_loaded?(AshPostgres.CustomExtension) do
       """
     end
 
+    defp add_money_neg do
+      """
+      CREATE OR REPLACE FUNCTION money_neg(money_1 money_with_currency)
+      RETURNS money_with_currency
+      IMMUTABLE
+      STRICT
+      LANGUAGE plpgsql
+      AS $$
+        DECLARE
+          currency varchar;
+          addition numeric;
+        BEGIN
+          currency := currency_code(money_1);
+          addition := amount(money_1) * -1;
+          return row(currency, addition);
+        END;
+      $$;
+
+      CREATE OPERATOR - (
+          rightarg = money_with_currency,
+          procedure = money_neg
+      );
+      """
+    end
+
+    defp remove_money_neg do
+      """
+      DROP OPERATOR -(none, money_with_currency);
+
+      DROP FUNCTION IF EXISTS money_neg(money_1 money_with_currency);
+      """
+    end
+
+    defp add_money_sub do
+      """
+      CREATE OR REPLACE FUNCTION money_sub(money_1 money_with_currency, money_2 money_with_currency)
+      RETURNS money_with_currency
+      IMMUTABLE
+      STRICT
+      LANGUAGE plpgsql
+      AS $$
+        DECLARE
+          currency varchar;
+          subtraction numeric;
+        BEGIN
+          IF currency_code(money_1) = currency_code(money_2) THEN
+            currency := currency_code(money_1);
+            subtraction := amount(money_1) - amount(money_2);
+            return row(currency, subtraction);
+          ELSE
+            RAISE EXCEPTION
+              'Incompatible currency codes for - operator. Expected both currency codes to be %', currency_code(money_1)
+              USING HINT = 'Please ensure both columns have the same currency code',
+              ERRCODE = '22033';
+          END IF;
+        END;
+      $$;
+
+      CREATE OPERATOR - (
+          leftarg = money_with_currency,
+          rightarg = money_with_currency,
+          procedure = money_sub,
+          commutator = -
+      );
+      """
+    end
+
+    defp remove_money_sub do
+      """
+      DROP OPERATOR - (money_with_currency, money_with_currency);
+
+      DROP FUNCTION IF EXISTS money_sub(money_1 money_with_currency, money_2 money_with_currency);
+      """
+    end
+
     defp add_money_mult do
       """
       CREATE OR REPLACE FUNCTION money_mult(multiplicator numeric, money money_with_currency)