Terraform: Copy specific elements from map where specified by list

I feel like this is really easy and obvious, but for the life of me I can’t figure out how I should be approaching it.

I have a list of environments I wish to build ["env1", "env2"…], and then I have a map that specifies the values used for a specific environment.

    "wanted_envs": ["env1", "env2"]
    "envs": {
        "env1": {
            "type": "prod",
            "backend": "cust-prod-backend-vpc",
            "backend_subnet": "192.168.0.0/22",
            "k8_pod_subnet": "192.168.35.16.0/20",
            "k8_service_subnet": "192.168.40.0/20",
            "gateway_subnet": "1.2.3.0/27",
            "dmz": "env1"
        },
        "env2": {
            "type": "prod",
            "backend": "cust-prod-backend-vpc",
            "backend_subnet": "192.168.0.0/22",
            "k8_pod_subnet": "192.168.35.16.0/20",
            "k8_service_subnet": "192.168.40.0/20",
            "gateway_subnet": "1.2.3.0/27",
            "dmz": "env2"
        },
        "env3": {
            "type": "prod",
            "backend": "cust-prod-backend-vpc",
            "backend_subnet": "192.168.0.0/22",
            "k8_pod_subnet": "192.168.35.16.0/20",
            "k8_service_subnet": "192.168.40.0/20",
            "gateway_subnet": "1.2.3.0/27",
            "dmz": "env3"
        },
        ......

I simply need to create a local variable, which contains the details of the environments I wish to build (listed in my list of strings) copied from the map in envs.

In my locals definition I’ve tried various forms of for_each thinking I can get the value from the list of required envs, but I can’t quite figure it out without just getting errors from doing the ‘wrong’ thing with terraform.

I would end up with a map, containing only the values for the envs listed in wanted_envs.

Like so:

    "envs_to_build": {
        "env1": {
            "type": "prod",
            "backend": "cust-prod-backend-vpc",
            "backend_subnet": "192.168.0.0/22",
            "k8_pod_subnet": "192.168.35.16.0/20",
            "k8_service_subnet": "192.168.40.0/20",
            "gateway_subnet": "1.2.3.0/27",
            "dmz": "env1"
        },
        "env2": {
            "type": "prod",
            "backend": "cust-prod-backend-vpc",
            "backend_subnet": "192.168.0.0/22",
            "k8_pod_subnet": "192.168.35.16.0/20",
            "k8_service_subnet": "192.168.40.0/20",
            "gateway_subnet": "1.2.3.0/27",
            "dmz": "env2"
        }
}

>Solution :

Using a for expression can get you what you need. Consider the following shortened example.

locals {
  envs = {
    env1 = {
      prop = "foo"
    },
    env2 = {
      prop = "bar"
    },
    env3 = {
      prop = "baz"
    }
  }

  targets = ["env1", "env3"]
}

output "target_envs" {
  value = [for k, v in local.envs : v if contains(local.targets, k)]
}

This yields the output of:

Changes to Outputs:
  + target_envs = [
      + {
          + prop = "foo"
        },
      + {
          + prop = "baz"
        },
    ]

And if you want to maintain the keys, as in your edit, one can do it like so:

locals {
  envs = {
    env1 = {
      prop = "foo"
    },
    env2 = {
      prop = "bar"
    },
    env3 = {
      prop = "baz"
    }
  }

  targets = ["env1", "env3"]
}

output "target_envs" {
  value = { for k, v in local.envs : k => v if contains(local.targets, k) }
}

which yields:

Changes to Outputs:
  + target_envs = {
      + env1 = {
          + prop = "foo"
        }
      + env3 = {
          + prop = "baz"
        }
    }