Do not use purchase_unit_price field as it does not exists anymore.

This commit refs #22596


(cherry picked from commit b0c17e9d91)

stock.py

@@ -3,9 +3,6 @@
 from trytond.pool import PoolMeta, Pool
 from trytond.model import fields
 from trytond.modules.product import price_digits
-from trytond.transaction import Transaction
-
-__all__ = ['Lot']
 
 
 class Lot(metaclass=PoolMeta):
@@ -14,18 +11,18 @@ class Lot(metaclass=PoolMeta):
     cost_price = fields.Function(fields.Numeric(
         'Cost Price', digits=price_digits), 'get_cost_price')
 
-    def get_cost_price(self, name=None):
+    def get_cost_price(self, name=None, date=None):
         pool = Pool()
         Move = pool.get('stock.move')
         Date = pool.get('ir.date')
 
-        date = Transaction().context.get('date') or Date.today()
+        if not date:
+            date = Date.today()
         moves = Move.search([
             ('lot', '=', self),
             ('state', 'in', ['assigned', 'done']),
             ('origin', 'like', 'purchase.line,%'),
             ('effective_date', '<=', date)
-        ])
-
+        ], order=[('effective_date', 'DESC')])
         if moves:
-            return moves[0].unit_price
+            return moves[0].origin.unit_price