## 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

addons/purchase/purchase.py
res = {}
        cur_obj=self.pool.get('res.currency')
        line_obj = self.pool['purchase.order.line']
        for order in self.browse(cr, uid, ids, context=context):
            res[order.id] = {
                'amount_untaxed': 0.0,
                'amount_tax': 0.0,
                'amount_total': 0.0,
            }
            val = val1 = 0.0
            cur = order.pricelist_id.currency_id
            for line in order.order_line:
                val1 += line.price_subtotal
                line_price = line_obj._calc_line_base_price(cr, uid, line,
                                                            context=context)
                line_qty = line_obj._calc_line_quantity(cr, uid, line,
                                                        context=context)
                for c in self.pool['account.tax'].compute_all(
                        cr, uid, line.taxes_id, line_price, line_qty,
                        line.product_id, order.partner_id)['taxes']:
                    val += c.get('amount', 0.0)
            res[order.id]['amount_tax']=cur_obj.round(cr, uid, cur, val)
            res[order.id]['amount_untaxed']=cur_obj.round(cr, uid, cur, val1)
            res[order.id]['amount_total']=res[order.id]['amount_untaxed'] + res[order.id]['amount_tax']
        return res
    def _set_minimum_planned_date(self, cr, uid, ids, name, value, arg, context=None):