[FIX] models: do not delete ir.model.fields in _field_create (#13565)

Following 5c5d0865b9, apparently unused records
are simply deleted after updating existing ones.  This causes an issue when a
module is installed or upgraded.  The inspection of `ir.model.fields` records
is based on a *partial* registry (not all modules loaded); therefore records
will be deleted and recreated afterwards.  This triggers inconsistencies in
other models that refer to `ir.model.fields`, like server actions, base action
rules, mail templates, etc.: many2one fields will be set to null!

openerp/models.py

@@ -443,7 +443,7 @@ class BaseModel(object):
                     raise UserError(_("Serialization field `%s` not found for sparse field `%s`!") % (f.serialization_field, k))
                 vals['serialization_field_id'] = serialization_field_id[0]
 
-            col = cols.pop(k, None)
+            col = cols.get(k)
             if not col:
                 cr.execute('select nextval(%s)', ('ir_model_fields_id_seq',))
                 id = cr.fetchone()[0]
@@ -472,11 +472,6 @@ class BaseModel(object):
                         cr.execute(query, vals)
                         break
 
-        # remove ir_model_fields that should not be there
-        if cols:
-            ids = tuple(col['id'] for col in cols.itervalues())
-            cr.execute("DELETE FROM ir_model_fields WHERE id IN %s", (ids,))
-
         self.invalidate_cache(cr, SUPERUSER_ID)
 
     @api.model