[FIX] fields: combine special values during onchange

Consider a many2one field `foo_id` on model `bar`, with an inverse one2many
field `bar_ids` on model `foo`.  During an onchange, the statement

    bar.foo_id = foo

puts a special value in cache to add `bar` to the value of `foo.bar_ids`
without explicitly reading `foo.bar_ids`.

Executing the above statement a second time, the cache of `foo.bar_ids` is no
longer empty.  This causes the actual value of `foo.bar_ids` to be read and
updated.  The issue is that this can be slow for large values of `foo.bar_ids`.

Avoid reading the value of the one2many field by handling the case where the
cache contains the special value: simply update the special value to take into
account the second assignment.

closes odoo/odoo#28982

1 files changed, 6 insertions, 0 deletions

diff --git a/odoo/api.py b/odoo/api.py
index 21095854a70..903e1187329 100644
--- a/odoo/api.py
+++ b/odoo/api.py
@@ -982,6 +982,12 @@ class Cache(object):
         value = self._data[field][record.id].get(key, SpecialValue(None))
         return default if isinstance(value, SpecialValue) else value
 
+    def get_special(self, record, field, default=None):
+        """ Return the special value of ``field`` for ``record``. """
+        key = field.cache_key(record)
+        value = self._data[field][record.id].get(key)
+        return value.get if isinstance(value, SpecialValue) else default
+
     def set_special(self, record, field, getter):
         """ Set the value of ``field`` for ``record`` to return ``getter()``. """
         key = field.cache_key(record)