set opportunity's party as default party value

__init__.py

@@ -7,5 +7,6 @@ from .activity import *
 
 def register():
     Pool.register(
+        Activity,
         SaleOpportunity,
         module='sale_opportunity_activity', type_='model')

activity.py

@@ -2,16 +2,20 @@
 # copyright notices and license terms.
 from trytond.model import fields
 from trytond.pool import PoolMeta, Pool
+from trytond.pyson import Eval
+from trytond.transaction import Transaction
 
 __metaclass__ = PoolMeta
-__all__ = ['SaleOpportunity']
+__all__ = ['Activity', 'SaleOpportunity']
 
 
 class SaleOpportunity:
     __name__ = 'sale.opportunity'
 
     activities = fields.One2Many('activity.activity', 'resource',
-        'Activities')
+        'Activities', context={
+            'opportunity_party': Eval('party'),
+            }, depends=['party'])
     last_action_date = fields.Function(fields.DateTime('Last Action'),
         'get_last_action_date')
 
@@ -24,3 +28,14 @@ class SaleOpportunity:
                 ],
             order=[('dtstart', 'desc')], limit=1)
         return act and act[0].dtstart or None
+
+
+class Activity:
+    __name__ = 'activity.activity'
+
+    @classmethod
+    def default_party(cls):
+        opportunity_party_id = Transaction().context.get('opportunity_party')
+        if opportunity_party_id:
+            return opportunity_party_id
+        return super(Activity, cls).default_party()