def check_networks_amount(self):
        netmanager = self.network_config['net_manager']
        net_size = int(self.network_config['fixed_network_size'])
        net_amount = int(self.network_config['fixed_networks_amount'])
        net_cidr = netaddr.IPNetwork(
            self.network_config['fixed_networks_cidr'])
        if not netmanager == consts.NOVA_NET_MANAGERS.FlatDHCPManager and\
                net_size * net_amount > net_cidr.size:
            self.err_msgs.append(
                u"Number of fixed networks ({0}) doesn't fit into "
                u"fixed CIDR ({1}) and size of one fixed network "
                u"({2}).".format(net_amount, net_cidr, net_size)
            )
            self.result.append({"ids": [],
                                "errors": ["fixed_network_size",
                                           "fixed_networks_amount"]})
        self.expose_error_messages()
Check that `fields` only refer to existing fields, doesn't contain
dataset/ETHPy150Open openstack/fuel-web/nailgun/nailgun/extensions/network_manager/checker.py/NetworkCheck.check_networks_amount--crossover with--dataset/ETHPy150Open django/django/django/contrib/admin/checks.py/BaseModelAdminChecks._check_fields