## Impacted versions:
 8.0 / 9.0 / 10.0
## Steps to reproduce:
* see https://github.com/odoo/odoo/issues/11841
* in a shell
  1. create an in memory record `order = self.env['purcahse.order'].new({....})`
  2. create an in memory record with a reference to the first one `order_line = self.env['purchase.order.line'].new({'order_id': order, ....})`
  3. order_line.order_id is an in memory recordset
  4. create a record for a model that inherits from mail.thread `self.env['product.product'].create({....})`
  5. At this stage order_line.order_id is an empty recordset

## Current behavior:
 In memory records are dropped when a call is done to self.invalidate_cache
## Expected behavior:
In memory records are preserved when a call is done to self.invalidate_cache


There are lot of places where the method `invalidate_cache` is called and the silent removal of the in memory records from all environments has unpredictable effects on fields modified by a computed method or an onchange or even related fields .... Moreover since all caches are invalidated each time you create / write / unlink  (https://github.com/odoo/odoo/blob/10.0/addons/mail/models/mail_followers.py#L111) an instance of a model that inherits from 'mail.thread', the performances of the server are severely penalized.

OPW703903

#
#
#
#
#
import pytz
from openerp import SUPERUSER_ID, workflow
from datetime import datetime
from dateutil.relativedelta import relativedelta
from operator import attrgetter
from openerp.tools.safe_eval import safe_eval as eval
from openerp.osv import fields, osv
from openerp.tools.translate import _
import openerp.addons.decimal_precision as dp
from openerp.osv.orm import browse_record_list, browse_record, browse_null
from openerp.tools import DEFAULT_SERVER_DATE_FORMAT, DEFAULT_SERVER_DATETIME_FORMAT, DATETIME_FORMATS_MAP
from openerp.tools.float_utils import float_compare
class purchase_order(osv.osv):
    def _amount_all(self, cr, uid, ids, field_name, arg, context=None):