[IMP] sale_margin_percentage: optimize migration script

There's a migration script to recompute sales marging on sale order
lines, which was raising memory errors with databases that contain many
lines.

The following optimizations are applied:
- Don't call compute method directly but mark the field to be computed.
  This avoids one write per record and makes the ORM to take care of
  batches.
- Read currency directly from the line, instead of from the pricelist
  which in turn requires reading the order.
- Don't read currency if quantity is zero as it won't be used anyway

sale_margin_percentage/migrations/15.0.1.0.1/post-migration.py

@@ -21,5 +21,6 @@ def update_margin_threshold(env):
 
 def recompute_margin_percent(env):
     order_lines = env["sale.order.line"].search([])
-    order_lines._compute_margin()
+    env.add_to_compute(field=order_lines._fields["margin_percent"], records=order_lines)
+    order_lines.flush()
     _logger.info("The margin percentage of %s records have been recomputed.", len(order_lines))

sale_margin_percentage/models/sale_order_line.py

@@ -13,11 +13,11 @@ class SaleOrderLine(models.Model):
     def _compute_margin(self):
         res = super()._compute_margin()
         for line in self:
-            currency = line.order_id.pricelist_id.currency_id
             if not line.product_uom_qty:
                 line.margin_percent = 0.0
                 continue
 
+            currency = line.currency_id
             if currency.is_zero(line.price_unit) or currency.is_zero(line.price_subtotal):
                 line.margin_percent = -1.0
                 continue