## 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

_name = 'product.template'
    _inherit = 'product.template'
    
    def _get_buy_route(self, cr, uid, context=None):
        
        buy_route = self.pool.get('ir.model.data').xmlid_to_res_id(cr, uid, 'purchase.route_warehouse0_buy')
        if buy_route:
            return [buy_route]
        return []
    def _purchase_count(self, cr, uid, ids, field_name, arg, context=None):
        res = dict.fromkeys(ids, 0)
        for template in self.browse(cr, uid, ids, context=context):
            res[template.id] = sum([p.purchase_count for p in template.product_variant_ids])
        return res
    _columns = {
        'purchase_ok': fields.boolean('Can be Purchased', help="Specify if the product can be selected in a purchase order line."),
        'purchase_count': fields.function(_purchase_count, string='# Purchases', type='integer'),
    }
    _defaults = {
        'purchase_ok': 1,
        'route_ids': _get_buy_route,
    }
    def action_view_purchases(self, cr, uid, ids, context=None):
        products = self._get_products(cr, uid, ids, context=context)
        result = self._get_act_window_dict(cr, uid, 'purchase.action_purchase_line_product_tree', context=context)
        result['domain'] = "[('product_id','in',[" + ','.join(map(str, products)) + "])]"
        return result
class product_product(osv.Model):