terraform：每次运行时强制创建 route_table_association

Question

aws_route_table_association每次我运行计划/应用时，都会强制重新创建两个资源：

Terraform will perform the following actions:

<= module.mon.data.aws_subnet_ids.snetid
      id:             <computed>
      ids.#:          <computed>
      tags.%:         "1"
      tags.Name:      "zencopl-mon-*"
      vpc_id:         "vpc-0xxxxxxxxxxxxxxf"

-/+ module.mon.aws_route_table_association.snet[0] (new resource required)
      id:             "rtbassoc-058a3a92f42c51c9b" => <computed> (forces new resource)
      route_table_id: "rtb-05401d41b7281d81f" => "rtb-05401d41b7281d81f"
      subnet_id:      "subnet-032a4ee6fc6ebe945" => "${data.aws_subnet_ids.snetid.ids[count.index]}" (forces new resource)

-/+ module.mon.aws_route_table_association.snet[1] (new resource required)
      id:             "rtbassoc-09858f67c89412e90" => <computed> (forces new resource)
      route_table_id: "rtb-05401d41b7281d81f" => "rtb-05401d41b7281d81f"
      subnet_id:      "subnet-0bd026945b213219d" => "${data.aws_subnet_ids.snetid.ids[count.index]}" (forces new resource)

看起来，这是因为我定义了数据源来获取子网 ID：

data "aws_subnet_ids" "snetid" {
  vpc_id     = "${var.vpc_ids[var.idx]}"
  depends_on = [ "aws_subnet.snets" ]
  tags = {
    Name = "${var.vpc_names[var.idx]}-${var.inst_role}-*"
  }
}

然后像这样使用：

locals {
   a_zones = ["${slice(data.aws_availability_zones.azs.names,0,2)}"]
}
#
resource "aws_route_table_association" "snet" {
  count          = "${length(local.a_zones)}"
  route_table_id = "${aws_route_table.rtb.id}"
  subnet_id      = "${data.aws_subnet_ids.snetid.ids[count.index]}"
  depends_on     = [ "aws_subnet.snets" ]
}
#
module "mon" {
  source         = "../../modules/core-network"
  idx            = "0"
  inst_role      = "mon"
  vpc_names      = "${module.vpc.vpc_names}"
  vpc_ids        = "${module.vpc.vpc_ids}"
  ......
  ......
}

我无法找出哪里出了问题（或者是不是一个错误）。我尝试使用， lifecycle { ignore_changes = [ .... ]}但无法弄清楚应该忽略什么。虽然它没有造成任何实际问题（从部署方面来看），但却造成了很多混乱，我真的很想修复它。有人能指出我做错了什么或遗漏了什么吗？提前谢谢！！

—桑