[FIX] base: avoid prefetching all the fields when getting context

When getting the context of res_users, all the fields are read but if
the schema is being modified and the modifications are not yet commited
in the database, this leads to a bad query.

With this commit, a read is used to fetch only the needed fields.

Thanks to @RCO for finding this issue that only occurs in specific
planetary alignment.

closes odoo/odoo#27263

odoo/addons/base/res/res_users.py

@@ -423,20 +423,19 @@ class Users(models.Model):
     @tools.ormcache('self._uid')
     def context_get(self):
         user = self.env.user
-        result = {}
-        for k in self._fields:
-            if k.startswith('context_'):
-                context_key = k[8:]
-            elif k in ['lang', 'tz']:
-                context_key = k
-            else:
-                context_key = False
-            if context_key:
-                res = getattr(user, k) or False
-                if isinstance(res, models.BaseModel):
-                    res = res.id
-                result[context_key] = res or False
-        return result
+        # determine field names to read
+        name_to_key = {
+            name: name[8:] if name.startswith('context_') else name
+            for name in self._fields
+            if name.startswith('context_') or name in ('lang', 'tz')
+        }
+        # use read() to not read other fields: this must work while modifying
+        # the schema of models res.users or res.partner
+        values = user.read(list(name_to_key), load=False)[0]
+        return {
+            key: values[name]
+            for name, key in name_to_key.items()
+        }
 
     @api.model
     @api.returns('ir.actions.act_window', lambda record: record.id)