Preserve list ordering when creating maps in Terraform 0.12?

Question

I have the following snippets in my configuration - the idea is to change current logic/syntax from 0.11 to 0.12. First, I am creating a map from lists,

my_vars = zipmap(
  var.foo_vars,
  flatten(data.terraform_remote_state.foo.*.outputs.some_id)
)

Then iterate over it to produce some key value pairs.

...
"var": [for key in keys(local.my_vars) :
  {
    name  = key
    value = lookup(local.my_vars, key)
  }
],
...

And here is the relevant tfvars configuration.

foo_vars = [
    "A",
    "B",
    "C"
]

The problem is that this logic doesn't seem to preserver order and I can't figure out a good way to make this happen. From what I understand, once you turn the lists into a map with zipmap, the order is recalculated. Is there anything that can be done to have the original order preserved?

I'm not tied to the current solution, so maybe there is a way to generate the key/values that doesn't require a map to be created first and can be done instead with only the two lists?

~ foo          = [
                        {
                            name  = "A"
                            value = "1"
                        },
                      - {
                          - name  = "B"
                          - value = "2"
                        },
                        {
                            name  = "C"
                            value = "3"
                        },
                      + {
                          + name      = "B"
                          + valueFrom = "2"
                        },
                    ]

user_281848 · Answer

The important thing here is that, as you've noticed, Terraform's map type is an unordered map which identifies elements only by their keys, not by permission. Therefore if you have a situation where you need to preserve the order of a sequence then a map is not a suitable data structure to use.

I have a suspicion that keeping things ordered may not actually be necessary to solve your underlying problem here, but I can't tell from the information you've shared what the real-world meaning of all of these values is, so I'm going to answer on the assumption that you do need to preserve the order. If you are working with ordered sequences only because you are creating multiple instances of a resource using count, I'd suggest that you consider using resource for_each instead, which may allow you to solve your underlying problem in a way that is not sensitive to the order of items in var.foo_vars.

Given two lists of the same length, you can produce a new list that combines the corresponding elements from each list by writing a for expression like this:

locals {
  my_vars = [
    for i, some_id in data.terraform_remote_state.foo.*.outputs.some_id : {
      name  = var.foo_vars[i]
      value = some_id
    }
  ]
}

The above relies on the fact that i index values from one list are correlated with the element of the same index in the other list, and so we can use the i from the data source instances to access the corresponding element of var.foo_vars.

Preserve list ordering when creating maps in Terraform 0.12?

1 Answers