db_Subnet_group = stack.add_resource(DBSubnetGroup(
    'DbSubnetGroup',
    DBSubnetGroupDescription='Subnet group for an internal DB',
    SubnetIds=[Ref(db_subnet1), Ref(db_subnet2)]
))

db_instance = stack.add_resource(DBInstance(
    'DbInstance',
    AllocatedStorage=10,
    DBInstanceClass='db.t2.micro',
    DBSubnetGroupName=Ref(db_Subnet_group),
    DBName='gnt',
    Engine='postgres',
    MasterUsername='postgres',
    MasterUserPassword='postgres',
    MultiAZ=True,
    VPCSecurityGroups=[Ref(db_instance_security_group)]
))