Creating a Map from Flatten in Terraform

Question

Here is my issue - I have a locals code block like the below. I need to create a Map from these values (these values are used in multiple places)

locals {
  object = {
    1 = {
      name = "val1"
      keys = ["val1"]
    }
    2 = {
      name = "val2"
      keys = ["val2", "val3", "val4"]
    }
  }
  associations = flatten(
    [
      for obj in local.object : [
        for association_key in obj.keys : {
          "${obj.name}-${association_key}" = {
            key             = obj.name
            association_key = association_key
          }
        }
      ]
    ]
  )
}

Which when I run a terraform plan outputting the above - I get:

testing = [
      + {
          + val1-val1 = {
              + association_key = "val1"
              + key             = "val1"
            }
        },
      + {
          + val2-val2 = {
              + association_key = "val2"
              + key             = "val2"
            }
        },
      + {
          + val2-val3 = {
              + association_key = "val3"
              + key             = "val2"
            }
        },
      + {
          + val2-val4 = {
              + association_key = "val4"
              + key             = "val2"
            }
        },
    ]

What I need to get however looks like this:

testing = [
          + val1-val1 = {
              + association_key = "val1"
              + key             = "val1"
            }
          + val2-val2 = {
              + association_key = "val2"
              + key             = "val2"
            }
          + val2-val3 = {
              + association_key = "val3"
              + key             = "val2"
            }
          + val2-val4 = {
              + association_key = "val4"
              + key             = "val2"
            }
    ]

The reason for this is that there are many repetitions in keys value from object and there needs to be a unique value in the map.

I have attempted the solution Here - but I have not been able to create the Map that I need.

Are you sure that's what you want? You can't achieve such a list — Paolo
– Paolo, Commented Apr 16 at 9:52
@Paolo - I think I might have solved it (ironically thanks to a ChatGPT Answer) - I changed flatten to merge and used the Splat and seems to create what I need. am testing now. — TheDemonLord
– TheDemonLord, Commented Apr 16 at 10:04

Martin Atkins · Accepted Answer · 2025-04-17 00:34:19Z

1

If you have a list of maps and all of the keys across all of the maps are unique, you can use merge to combine them all into a single map with all of those keys represented.

For example:

merge(local.associations...)

The ... here tells Terraform that it should treat each element of local.associations as a separate argument to merge. Without that suffix Terraform would instead treat the entire list as just a single argument to merge, which would not work because merge expects all of its arguments to be maps.

This follows the same approach as one of the examples that is included in the documentation for merge at the time I'm writing this answer:

The following example uses the expansion symbol (...) to transform the value into separate arguments. Refer to Expanding Function Argument for details.
> merge([{a="b", c="d"}, {}, {e="f", c="z"}]...)
{
  "a" = "b"
  "c" = "z"
  "e" = "f"
}

answered Apr 17 at 0:34

Martin Atkins

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1 Comment

TheDemonLord Apr 17 at 1:48

Yep - you are right, it was Merge that I was after, not Flatten. Thanks for the answer :)

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